feat: add observability improvements (metrics, audit log, structured logging)

- Add Prometheus metrics package (internal/metrics) with deployment,
  container health, webhook, HTTP request, and audit counters/histograms
- Add audit_log SQLite table via migration 007
- Add AuditEntry model with CRUD operations and query methods
- Add audit service (internal/service/audit) for recording user actions
- Instrument deploy service with deployment duration, count, and
  in-flight metrics; container health gauge updates on deploy completion
- Instrument webhook service with event counters by app/type/matched
- Instrument HTTP middleware with request count, duration, and response
  size metrics; also log response bytes in structured request logs
- Add audit logging to all key handler operations: login/logout, app
  CRUD, deploy, cancel, rollback, restart/stop/start, webhook receipt,
  and initial setup
- Add GET /api/audit endpoint for querying recent audit entries
- Make /metrics endpoint always available (optionally auth-protected)
- Add comprehensive tests for metrics, audit model, and audit service
- Update existing test infrastructure with metrics and audit dependencies
- Update README with Observability section documenting all metrics,
  audit log, and structured logging

internal/database/migrations/007_add_audit_log.sql

@@ -0,0 +1,16 @@
+-- Audit log table for tracking user actions.
+CREATE TABLE audit_log (
+    id INTEGER PRIMARY KEY,
+    user_id INTEGER,
+    username TEXT NOT NULL,
+    action TEXT NOT NULL,
+    resource_type TEXT NOT NULL,
+    resource_id TEXT,
+    detail TEXT,
+    remote_ip TEXT,
+    created_at DATETIME DEFAULT CURRENT_TIMESTAMP
+);
+
+CREATE INDEX idx_audit_log_created_at ON audit_log(created_at);
+CREATE INDEX idx_audit_log_action ON audit_log(action);
+CREATE INDEX idx_audit_log_resource ON audit_log(resource_type, resource_id);