Fix audit findings: lock architecture, security, deployment hardening

Close all 14 findings from the codebase audit (1 high, 5 medium, 8 low):

- Locking: replace the global API and workspace mutexes with per-instance
  and per-user keyed locks (new internal/api/locks.go) so a slow restore,
  upgrade or transfer no longer blocks unrelated users or instances; move
  Docker I/O out of the build queue and invalidate concurrent image-index
  observations with a generation counter; serialize instance deletion
  behind in-flight creation.
- Security: equalize login timing for unknown usernames, periodically
  purge expired sessions/refresh tokens/lock rows, URL-encode orphan IDs
  in the frontend, upgrade vitest 3.x->5.x to clear 3 npm audit
  vulnerabilities (2 critical RCE).
- Deployment: add a -healthcheck mode probing /healthz (used by Dockerfile
  HEALTHCHECK and compose), make ADMIN_PASSWORD optional in compose with
  removal guidance after first start, exclude test scripts from the image,
  and cap panel resources (cpus: 2, pids_limit: 512).
- Reliability: check rows.Err() on admin listing loops, surface intent
  polling errors without sticking, disable close during in-flight create.

Verified: go build/vet, full go test -race -count=1 ./..., frontend
tsc build + 32 tests, npm audit 0, compose config, real-container
healthcheck.

.env.example

@@ -1,7 +1,11 @@
 # Copy to .env (ignored by git). Use an absolute dedicated data directory.
 DSH_HOST_DATA_ROOT=/var/lib/dsh-mgr
 DSH_ADMIN_USERNAME=admin
-# Supply a unique strong password before starting; no default credential.
+# Supply a unique strong password before FIRST start; the panel reads it only
+# while the account table is empty. Remove the value after the administrator
+# account exists, then run `docker compose up -d --force-recreate panel`
+# to remove it from the container environment and `docker inspect` output.
+# Restarting the existing container does not update its environment.
 DSH_ADMIN_PASSWORD=
 # Numeric group from: stat -c %g /var/run/docker.sock
 DOCKER_GID=
@@ -10,6 +14,8 @@ DSH_DOCKER_ROOT_DIR=
 # Optional exact extension allowlist, e.g. .txt,.md,.json,.yaml,.go
 # Empty permits arbitrary workspace files; this is not malware/content scanning.
 UPLOAD_ALLOWED_EXTENSIONS=
-# Optional proxy networks. X-Forwarded-For is ignored unless the peer is in these CIDRs.
+# Public origin of the panel exactly as browsers see it (scheme://host, no
+# path); controls Origin/CSRF validation and Secure cookies.
 PANEL_PUBLIC_URL=http://127.0.0.1:8088
+# Optional proxy networks. X-Forwarded-For is ignored unless the peer is in these CIDRs.
 TRUSTED_PROXY_CIDRS=