Add HTTP service hardening policy for 1.0 releases (#17)

Closes #16

Adds a comprehensive HTTP/web service security hardening policy to `REPO_POLICIES.md` that must be satisfied before tagging 1.0. The policy covers all items sneak specified (without limitation):

**Security headers** — HSTS (min 1 year, includeSubDomains), CSP (restrictive `default-src 'self'` baseline), X-Frame-Options / frame-ancestors, X-Content-Type-Options: nosniff, Referrer-Policy, Permissions-Policy.

**Request/response limits** — max request body size on all endpoints, max response size for paginated APIs, ReadTimeout + ReadHeaderTimeout (slowloris defense), WriteTimeout, IdleTimeout, per-handler execution time limits.

**Authentication & session security** — rate limiting on password-based auth (API keys exempt as high-entropy), CSRF tokens on state-mutating forms (header-auth APIs exempt), bcrypt/scrypt/argon2 for passwords, session cookies with HttpOnly + Secure + SameSite.

**Reverse proxy awareness** — true client IP detection via X-Forwarded-For/X-Real-IP with trusted proxy allowlist (never trust unconditionally).

**CORS** — explicit origin allowlist for authenticated endpoints; wildcard only for public unauthenticated read-only APIs.

**Error handling** — no leaking stack traces, SQL queries, file paths, or implementation details to clients.

**TLS** — HSTS and secure cookie flags required regardless of whether the service terminates TLS directly or sits behind a reverse proxy.

The policy is explicitly non-exhaustive (defense-in-depth: "when in doubt, harden").

Also adds corresponding checklist sections to `EXISTING_REPO_CHECKLIST.md` and `NEW_REPO_CHECKLIST.md` so that HTTP hardening is verified during repo setup and 1.0 preparation.

Co-authored-by: user <user@Mac.lan guest wan>
Co-authored-by: clawbot <clawbot@eeqj.de>
Reviewed-on: #17
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

prompts/EXISTING_REPO_CHECKLIST.md

@@ -1,6 +1,6 @@
 ---
 title: Existing Repo Checklist
-last_modified: 2026-02-22
+last_modified: 2026-03-10
 ---
 
 Use this checklist when beginning work in a repo that may not yet conform to our
@@ -78,6 +78,22 @@ with your task.
       `internal/`, `static/`, etc.)
 - [ ] Go migrations in `internal/db/migrations/` and embedded in binary
 
+# HTTP Service Hardening (if targeting 1.0 and the repo is an HTTP/web service)
+
+- [ ] Security headers set on all responses (HSTS, CSP, X-Frame-Options,
+      X-Content-Type-Options, Referrer-Policy, Permissions-Policy)
+- [ ] Request body size limits enforced on all endpoints
+- [ ] Read/write/idle timeouts configured on the HTTP server (slowloris defense)
+- [ ] Per-handler execution time limits in place
+- [ ] Password-based auth endpoints are rate-limited
+- [ ] CSRF tokens on all state-mutating HTML forms
+- [ ] Passwords hashed with bcrypt, scrypt, or argon2
+- [ ] Session cookies use HttpOnly, Secure, and SameSite attributes
+- [ ] True client IP correctly detected behind reverse proxy (trusted proxy
+      allowlist configured)
+- [ ] CORS restricted to explicit origin allowlist for authenticated endpoints
+- [ ] Error responses do not leak stack traces, SQL queries, or internal paths
+
 # Final
 
 - [ ] `make check` passes