fix(omniroute): add required WS bridge secret, memory ceiling, shutdown grace period

Cross-checked the deployment against OmniRoute's own docs
(docs/reference/ENVIRONMENT.md, docs/guides/DOCKER_GUIDE.md) and found
three gaps from the original migration:

- OMNIROUTE_WS_BRIDGE_SECRET was entirely missing - ENVIRONMENT.md marks
  it REQUIRED (production), for the internal Codex Responses WebSocket
  bridge. docker compose config validated fine without it (compose
  doesn't know omniroute's own required-var list), so this went
  unnoticed until checking the docs directly.
- No mem_limit/OMNIROUTE_MEMORY_MB - the Docker guide is explicit that
  the 1024MB default heap is dashboard-only sized; coding-agent workloads
  (every client this stack has) need OMNIROUTE_MEMORY_MB=8192 and a
  10+ GiB container ceiling. Set both.
- No stop_grace_period - the guide's --stop-timeout 40 equivalent, so
  SQLite WAL changes checkpoint back into the main DB file on shutdown
  instead of getting killed mid-write.

Redis checked and confirmed correctly absent - OmniRoute uses SQLite
only, no Redis anywhere in its docs.

Still open: whether API_PORT actually isolates /dashboard and /api/*
from the published port, or bridges everything through (see issue #31)
- OmniRoute's own ARCHITECTURE.md doesn't document split-port mode as a
real security boundary, and the live "[API Bridge] ... -> dashboard"
log line is ambiguous. Waiting on a live curl test against
proxy-ai.home before treating that as resolved.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VPZ6TogJiYxG8E4EQBB197

.env.example

@@ -63,6 +63,9 @@ OMNIROUTE_STORAGE_ENCRYPTION_KEY=
 # Per-deployment salts — random is fine, just needs to be stable:
 OMNIROUTE_MACHINE_ID_SALT=
 OMNIROUTE_CLI_SALT=
+# Required (production) — shared secret for the internal Codex Responses
+# WebSocket bridge. Random value, filled in automatically:
+OMNIROUTE_WS_BRIDGE_SECRET=
 # Per-workload virtual keys (one per client that calls the gateway) have no
 # scripted /key/generate equivalent yet — omniroute's key-creation endpoint
 # needs a dashboard login session, not a static bearer key (see issue #37).