fix(admin-security): SSRF defence + CSRF fail-closed + analysis .no_proxy() (0.87.2)

Four high/medium findings from the audit, plus their tests:

1. **SSRF on admin-editable URLs.** `CrawlerSettings::start_url` and
   `AnalysisSettings::endpoint` validated only with `Url::parse`. A
   hostile or CSRF-able admin could repoint at `169.254.169.254`
   (cloud metadata), `127.0.0.1:5432` (postgres), or any RFC1918 host
   — and the vision worker bearer-attaches an env-managed API key to
   every call. Extract `ensure_public_target` from `safety::is_safe_url`
   (allowlist-free public-host check: scheme + private-IP literal +
   localhost) and wire it into both fields. Docker DNS names
   (`mangalord-vision`) keep passing because they're hostnames, not
   IP literals. The analysis check is gated on `enabled=true` so the
   dev-default `localhost:8000` stays usable until the worker is
   actually turned on (toggling enabled=true later re-runs the gate).

2. **Admin CSRF fail-open default.** `ADMIN_ALLOWED_ORIGINS=` empty
   silently skipped the entire CSRF check, so an operator forgetting
   the env var shipped an unguarded admin surface. Cookie-auth POSTs
   now fail-closed when the allowlist is empty AND when neither
   `Origin` nor `Referer` accompanies the request. Two carve-outs:
   `Authorization: Bearer …` callers bypass (bots can't be CSRF'd),
   and requests with NO session cookie at all bypass to let the auth
   extractor return a clean 401 instead of a confusing 403.

3. **Analysis HTTP clients didn't `.no_proxy()`.** The crawler client
   already did. Ambient `HTTP_PROXY`/`HTTPS_PROXY` in container env
   would exfiltrate page-image bytes + the bearer key through an
   upstream proxy. Same for the readiness probe.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/tests/api_admin_settings.rs

@@ -237,7 +237,15 @@ async fn put_analysis_triggers_reload_and_flips_gate(pool: PgPool) {
         .clone()
         .oneshot(common::put_json_with_cookie(
             "/api/v1/admin/settings/analysis",
-            json!({ "enabled": true, "model": "qwen2-vl", "temperature": 0.4 }),
+            // Endpoint must pass `ensure_public_target` whenever
+            // `enabled=true`. Use the documented docker-internal DNS
+            // name; a literal IP/localhost would (rightly) 422.
+            json!({
+                "enabled": true,
+                "endpoint": "http://mangalord-vision:8000/v1/chat/completions",
+                "model": "qwen2-vl",
+                "temperature": 0.4,
+            }),
             &cookie,
         ))
         .await