Close the /admin/features side-door that needed a container shell to flip AI spend

Board minutes #62 Decision 2 (server-26#64), due 2026-08-31. CTO draft #60
finding 1 and CISO draft #61 finding 3 reached this independently.

GET/PUT /admin/features accepted only a Firebase admin token, so the unattended
runbook had no headless path and SSHed into the c2-core container to write
config/ai_features with the admin SDK. Moving a platform-wide AI cost switch
required a full container shell, and set_flags() wrote no audit entry either
way, so a flag flip was unattributable however it happened.

- New agent_service_key (AGENT_SERVICE_KEY), deliberately separate from the
  Discord bot's service_key. Sharing one key would collapse two principals into
  a single unattributable identity in every log line, and the bot has no
  business flipping AI flags regardless.
- require_agent_key_or_admin accepts the agent key or a Firebase admin, and
  rejects the Discord key. The "key is configured" guard is load-bearing:
  compare_digest("", "") is a match, so a deployment that never set the key
  would otherwise accept an empty credential.
- set_flags() writes an audit_log entry with before/after values and the actor,
  wrapped so an audit failure cannot lose the flag write or 500 the route.
- Cascade helper sets the global doc and every system carrying an ai_flags
  override in one call. A global False already beats everything, but a system
  False beats a global True, so turning AI *on* could half-apply and leave a
  radio system hot after shutoff. It scans for the override rather than
  hardcoding the two known system IDs, so a new system cannot silently defeat
  it.
- cascade defaults to False. PUT /systems/{id}/ai-flags and the AiFlagsPanel
  toggle mean a per-system override is deliberate operator intent; cascading by
  default would erase it on any unrelated global flip. The runbook opts in.

Issue items 5 and 6 (retiring the SSH path from drb-worksession.md) are NOT
done here and the runbook is untouched. The credential does not exist in
production yet, so the SSH path is still the only one that works; retiring it
now would break the next unattended run. Owner activation is recorded on #64.

Tests 273 -> 289.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docker-compose.yml

@@ -19,6 +19,14 @@ services:
       - mosquitto_data:/mosquitto/data
       - mosquitto_certs:/mosquitto/certs
 
+  # c2-core takes ALL of its configuration from ./drb-c2-core/.env — there is
+  # deliberately no `environment:` block here. An entry in that block wins over
+  # env_file, so listing a key here (e.g. AGENT_SERVICE_KEY=${AGENT_SERVICE_KEY})
+  # would let an unset top-level .env silently blank out a value the owner had
+  # correctly pasted into drb-c2-core/.env. New settings go in
+  # drb-c2-core/.env.example and, for the VM, in
+  # infra/ansible/roles/deploy/templates/c2-core.env.j2 + vault.yml.
+  # AGENT_SERVICE_KEY (server-26#64) is configured that way.
   c2-core:
     image: ${REGISTRY}/c2-core:${TAG:-latest}
     build: ./drb-c2-core