Harden operator-set target headers (closes #233)

Three findings from the review of the per-target request headers
feature.

Configured headers no longer follow a redirect off the origin the
target names. net/http withholds only Authorization and Cookie
across a host change, so an operator's X-Api-Key or PRIVATE-TOKEN
would follow a 302 to a host they never configured. Redirects are
still followed — refusing them would break every destination that
legitimately redirects and would record the 3xx as the delivery's
result — but a hop to another host, another port, or down from
https to http drops every header the target configured. The shared
SSRF-safe transport is kept on that client, so each hop is still
dialled through the private-IP guard.

Trailer joins the reserved names. net/http strips it from the
request it writes, so a configured one was accepted, stored, and
provably never sent.

The invalid-header-name error no longer quotes the text before the
first colon. That text is only a name if it parses as one; when it
does not, a pasted value whose own colon split the line put half a
token into a 400 body. TestParseTargetHeaders_ErrorsNeverQuoteAValue
asserted this invariant while only exercising the after-the-colon
case, and now covers the before-the-colon one.

README documents the http target's config keys, the 300-second
timeout ceiling, the reserved-header list and the redirect
behaviour; the edit form's hint gains Trailer and the redirect note.

README.md

@@ -1049,6 +1049,41 @@ events should be forwarded.
 The `config` field stores type-specific configuration as JSON (e.g.,
 destination URL, custom headers, timeout settings).
 
+**`http` target configuration:**
+
+| Key       | Type          | Description |
+| --------- | ------------- | ----------- |
+| `url`     | string        | Destination the event is POSTed to |
+| `headers` | object        | Extra request headers, applied last so they win over the event's own forwarded headers |
+| `timeout` | integer (sec) | Per-target request timeout; unset (or 0) uses the shared 30-second client timeout |
+
+`timeout` is capped at **300 seconds**, and the form rejects anything
+above it rather than substituting the cap. A delivery attempt holds one
+of the bounded pool's workers for its whole duration, so an unbounded
+timeout would let a single unresponsive destination stall the queue.
+
+`headers` rejects the names the delivery path or `net/http` writes
+regardless of what is configured: `Host`, `Content-Length`,
+`Transfer-Encoding`, `Connection`, `Trailer` and `User-Agent`. These are
+refused at the form rather than accepted and ignored, because a stored
+header that provably never reaches the wire tells the operator their
+configuration took effect when it did not. `Content-Type` is _not_
+reserved: a configured one deliberately overrides the event's.
+
+**Redirects.** A redirect from an `http` target's destination is
+followed, up to ten hops, and the delivery's recorded status and body
+come from the final hop. Configured `headers` are dropped as soon as a
+hop leaves the origin the target names — a different host, a different
+port, or a step down from `https` to `http` — because a configured
+header is routinely a credential (`X-Api-Key`, `PRIVATE-TOKEN`) and an
+open redirect at the destination would otherwise hand it to a host the
+operator never chose. `net/http` already does this for `Authorization`
+and `Cookie`. Redirects within the target's own origin keep every
+configured header, so a destination that redirects its own paths is
+unaffected. Each hop is dialled through the same SSRF guard as the
+first, so a redirect aimed at a private or reserved address is refused
+at connect time.
+
 #### APIKey
 
 A programmatic access credential for API authentication.