State the UUID-is-the-credential rule as a rule (closes #301) (#302)

Closes #301. Docs-only apart from one test comment; no behaviour change.

The receiver has authenticated on the entrypoint UUID alone since inbound signature verification was removed in #279. The README described that as the current state. It did not say it is the decision, which leaves a future contributor free to propose HMAC as an improvement rather than as a reversal.

What changed:

- `## The entrypoint URL is the authentication secret` now states the rule: the v4 UUID at `/webhook/{uuid}` is the credential and the only one; no shared secret, HMAC signature, bearer token or second factor will be added, including as defence in depth. It names the removal that settled it, and it says explicitly that signature headers a sender sends anyway are stored and forwarded but never checked — the previous text left that ambiguous.
- The same section carries the two consequences an operator has to act on: the URL is a capability, so keep it out of logs, tickets and screenshots; and rotation means minting a new entrypoint, not changing a key.
- It also handles the case the rule will next be argued from: a sender that only supports signed payloads to a well-known URL is a constraint on that integration, to be raised on its own terms, not grounds to reintroduce shared secrets.
- The rule is reachable without scrolling 1,100 lines: a pointer in the intro, a new first bullet under Authentication (which previously listed the web UI, the API and `/metrics` and said nothing about the receiver at all), and a sharpened bullet under Security.

Stale language found and corrected: one, in `internal/delivery/redirect_test.go`. Its comment justified same-origin header retention partly by "the inbound signature the receiver verifies" — in this repo's vocabulary "the receiver" is `/webhook/{uuid}`, which verifies nothing. The endpoint that verifies it is the delivery target's, and the comment now says so.

Two places that read like stale signing language were checked and left alone as accurate: `internal/delivery/redirect.go` and `internal/server/sentry.go` describe signature headers senders put on the receiver route, which do arrive and are forwarded — neither claims webhooker checks them.

`REPO_POLICIES.md` was deliberately not touched. It is the cross-project policy document synced from `sneak/prompts` and carries `last_modified` front matter for that purpose, so a webhooker-specific carve-out does not belong in it. Worth knowing: its hardening section ends "if a standard security hardening measure exists for HTTP services and is not listed here, it is still expected. When in doubt, harden" — that is the sentence a future HMAC proposal will cite, and only the README now answers it.

`TODO.md` is untouched per its own Workflow section (issue branches do not touch it).

Co-authored-by: sneak <sneak@sneak.berlin>
Reviewed-on: #302
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

README.md

@@ -7,6 +7,13 @@ services, durably stores them, and delivers them to configured targets
 with retry support, logging, and observability. Category: infrastructure
 / web service. License: MIT.
 
+Each entrypoint is a version 4 UUID served at `/webhook/{uuid}`, and
+that UUID is the entrypoint's only credential. webhooker does not use
+shared secrets, HMAC signatures or token headers on the receiver, and
+will not add them — read
+[The entrypoint URL is the authentication secret](#the-entrypoint-url-is-the-authentication-secret)
+before deploying one.
+
 ## Getting Started
 
 ### Prerequisites
@@ -1149,14 +1156,38 @@ backups at rest and restrict who can read them.
 
 ## The entrypoint URL is the authentication secret
 
-The receiver verifies nothing about an inbound request. The UUID in an
+**The entrypoint UUID is the credential, and it is the only one.**
-entrypoint's URL is its credential: anyone who holds that URL can
+webhooker mints a version 4 UUID per entrypoint and serves it at
-submit events to it, and the receiver checks nothing else about the
+`/webhook/{uuid}`. Possession of that URL is the authentication:
-sender. Treat an entrypoint URL the way you would treat an API token.
+anyone who holds it can submit events to the entrypoint, and the
+receiver verifies nothing else about the sender.
 
-There is no way to rotate the UUID in place. To retire one, delete the
+There is no shared secret, no HMAC signature, no bearer token and no
-entrypoint (or deactivate it, which answers `410`) and create a new
+second factor on the receiver, and none will be added. This was
-one, then point the sender at the new URL.
+considered and rejected; the implementation that existed was removed
+in [PR #279](https://git.eeqj.de/sneak/webhooker/pulls/279), closing
+[issue #67](https://git.eeqj.de/sneak/webhooker/issues/67) and
+[issue #241](https://git.eeqj.de/sneak/webhooker/issues/241). A
+proposal to reintroduce any of them — including as "defence in depth"
+alongside the UUID — is answered by this section. Inbound signature
+headers a sender sends anyway (`X-Hub-Signature` and its
+per-provider equivalents) are stored and forwarded as ordinary
+headers; nothing checks them.
+
+What that means for an operator:
+
+- **The URL is a capability, so treat it as a secret.** Keep it out of
+  logs, ticket bodies, chat messages and screenshots. Anyone who reads
+  it anywhere can post events as that sender.
+- **Rotating means minting a new entrypoint, not changing a key.**
+  There is no way to rotate the UUID in place. To retire one, delete
+  the entrypoint (or deactivate it, which answers `410`) and create a
+  new one, then point the sender at the new URL.
+- **A sender that cannot be given a secret URL is a constraint on that
+  integration, not a reason to change this.** If a service only
+  supports signed payloads to a well-known URL, raise it as its own
+  problem — pick a different integration path, or accept that it
+  cannot be used. It is not grounds to reintroduce shared secrets.
 
 ## Entrypoints
 
@@ -2867,6 +2898,10 @@ check, see [The login endpoint](#the-login-endpoint).
 
 ### Authentication
 
+- **Webhook receiver:** the entrypoint UUID in the URL, and nothing
+  else. No shared secret, no HMAC signature, no token header, and none
+  will be added — see
+  [The entrypoint URL is the authentication secret](#the-entrypoint-url-is-the-authentication-secret).
 - **Web UI:** Cookie-based sessions using gorilla/sessions with
   encrypted cookies. Sessions are configured with HttpOnly, SameSite
   Lax, and Secure whenever the request is on TLS — the flag follows the
@@ -2906,7 +2941,8 @@ check, see [The login endpoint](#the-login-endpoint).
   mode
 - **The entrypoint URL is the receiver's only credential.** Nothing
   about an inbound request is verified; possession of the UUID
-  authorises submission (see
+  authorises submission, and no shared secret or signature check will
+  be added alongside it (see
   [The entrypoint URL is the authentication secret](#the-entrypoint-url-is-the-authentication-secret))
 - **SSRF prevention** for HTTP delivery targets: private/reserved IP
   ranges (RFC 1918, loopback, link-local, cloud metadata) are blocked

internal/delivery/redirect_test.go

@@ -170,7 +170,8 @@ func TestDelivery_CrossOriginRedirectDropsOriginScopedHeaders(
 // Stripping must not fire within the configured origin, or every
 // destination that redirects its own path would lose its
 // credential and start answering 401 — and would lose the inbound
-// signature the receiver verifies.
+// signature header the target endpoint verifies. webhooker's own
+// receiver verifies no signature; it only forwards the header.
 func TestDelivery_SameOriginRedirectKeepsOriginScopedHeaders(
 	t *testing.T,
 ) {