v0.18.0 Slice 3: webhook tightening — mandatory secret + dev-bypass

`GITEA_WEBHOOK_SECRET` is now mandatory at startup. The framework
refuses to load_config() when the env var is empty unless the
operator opts into the dev-bypass with `RFC_APP_INSECURE_WEBHOOKS=1`.
This is the v0.18.0 startup-loud-failure shape — the pre-v0.18.0
"silently accept unsigned POSTs when the secret is empty" path is
the bug the proposal targets.

`webhooks.receive`:
  * Defense in depth: refuses 500 if the secret is empty at request
    time and the dev-bypass is not set (catches the case where
    something mutates env after startup).
  * Logs a loud warning every time a webhook lands under the
    bypass — so a misconfigured production deployment shows up in
    the logs even if the operator missed the warning at boot.
  * Adds an INFO log at the unknown-repo branch (previously
    silently 200-OK'd a hook on a fork or a stale Gitea binding).

`tmp_env` fixture binds a fake secret so the existing 277 tests
boot cleanly; the new test_webhooks_vertical.py exercises both
the production-secret path (valid signature, invalid signature,
missing signature) and the dev-bypass path (config loads with
empty secret when bypass set, refuses without it).

7 new tests; full suite: 284 passed.

backend/app/webhooks.py

@@ -12,6 +12,7 @@ import hashlib
 import hmac
 import json
 import logging
+import os
 
 from fastapi import APIRouter, Header, HTTPException, Request
 
@@ -40,7 +41,27 @@ def make_router(config: Config, gitea: Gitea) -> APIRouter:
         x_gitea_signature: str = Header(default=""),
     ):
         body = await request.body()
-        if config.webhook_secret:
+        # v0.18.0: defense in depth. config.py refuses to start
+        # when the secret is empty unless `RFC_APP_INSECURE_WEBHOOKS=1`
+        # is set; this branch catches the dev-bypass case (the only
+        # path where `config.webhook_secret` can be empty) and surfaces
+        # it loudly to the client. A POST that lands here with an
+        # empty secret on a production deployment indicates a
+        # mis-configuration (somebody flipped the bypass in prod),
+        # and the loud 500 is the proposal's whole point.
+        insecure = os.environ.get("RFC_APP_INSECURE_WEBHOOKS", "").strip() == "1"
+        if not config.webhook_secret:
+            if not insecure:
+                log.error(
+                    "webhook receiver misconfigured: GITEA_WEBHOOK_SECRET is empty "
+                    "and RFC_APP_INSECURE_WEBHOOKS=1 is not set"
+                )
+                raise HTTPException(status_code=500, detail="Webhook receiver misconfigured")
+            log.warning(
+                "webhook receiver running with RFC_APP_INSECURE_WEBHOOKS=1 — "
+                "signature verification is DISABLED. Production deployments MUST NOT set this."
+            )
+        else:
             if not _verify_signature(body, x_gitea_signature, config.webhook_secret):
                 raise HTTPException(status_code=401, detail="Invalid signature")
 
@@ -68,6 +89,17 @@ def make_router(config: Config, gitea: Gitea) -> APIRouter:
                 slug = _slug_for_repo(repo_full)
                 if slug:
                     await cache.refresh_rfc_repo(config, gitea, slug)
+                else:
+                    # v0.18.0: the proposal's "unknown-repo logging"
+                    # gesture — a hook on a fork or a stale repo binding
+                    # used to silently 200-OK here, hiding the
+                    # misconfiguration. Now the operator sees it in
+                    # the log.
+                    log.info(
+                        "webhook received for unknown repo: repo_full=%s event=%s "
+                        "(no cached_rfcs row matched; hook may be on a fork or stale)",
+                        repo_full, event,
+                    )
         except Exception:
             log.exception("webhook refresh failed")
             raise HTTPException(status_code=500, detail="Refresh failed")