fix(security): self-healing secrets + auth/SSRF/header hardening (v0.16.10-beta)

Follow-up to the /terminal authorization fix: a full-codebase security
review surfaced several more issues, fixed here. All changes are designed
to roll out with no interruption to existing deployments — no forced
migration, no manual re-key, no boot failure.

Self-healing secret management (utils/secrets.ts):
- JWT_SECRET / ENCRYPTION_KEY auto-generate to strong values on first boot
  and persist to the app_data volume (0600, outside the DB) when not set
  to a strong value in the environment. Replaces the prior behavior of
  running on the public repo defaults.
- decrypt() tries the current key plus every legacy/default key, so
  existing device credentials keep decrypting after a key change; a
  startup sweep re-encrypts stale rows forward (db/reencryptCredentials.ts).
- verifyToken() accepts the current signing secret plus prior *strong*
  secrets (never the public defaults), so rotation doesn't invalidate live
  sessions; sessions signed with a default secret require one re-login.
- Dockerfile pre-creates /app/data as the app user so the non-root
  process can persist to a fresh named volume; app_data volume added to
  both compose files; compose secret defaults changed to empty (auto-gen).

Authorization / input:
- Default Socket.IO namespace now requires a valid JWT (was unauthenticated
  and broadcasting fleet-activity events); frontend sends the token and
  drops the shared socket on logout.
- Webhook delivery blocks SSRF to loopback/private/link-local/CGNAT ranges
  and pins the request to the validated IP (WebhookService.ts).
- Rate limit added to /auth/totp/verify; global mutation backstop on /api.
- Password policy (>=10 chars, letter+digit) enforced on set/change/create.
- system.ts used verifyToken as middleware (always 500); now requireAuth.

Hardening:
- CSP + HSTS/nosniff/frame-deny headers in nginx, scoped to allow the
  OpenStreetMap tile/geocoding hosts the maps use.

Docs/tests:
- README secret section rewritten for the self-healing model.
- New unit tests for secret resolution, persistence, rotation, and
  legacy-key decryption (97 tests pass).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/Dockerfile

@@ -27,8 +27,10 @@ RUN apk add --no-cache arp-scan libcap openssl \
 # Copy built files
 COPY --from=builder /app/dist ./dist
 
-# Create backups directory
-RUN mkdir -p /app/backups
+# Create writable data directories. Pre-creating them (owned by appuser after
+# the chown below) means a fresh named volume mounted here inherits appuser
+# ownership, so the non-root process can persist backups and secrets.
+RUN mkdir -p /app/backups /app/data
 
 # Create non-root user
 RUN addgroup -S appgroup && adduser -S appuser -G appgroup