feat: implement pngx-controller with Gitea CI/CD deployment

- Full FastAPI sync engine: master→replica document sync via paperless REST API
- Web UI: dashboard, replicas, logs, settings (Jinja2 + HTMX + Pico CSS)
- APScheduler background sync, SSE live log stream, Prometheus metrics
- Fernet encryption for API tokens at rest
- pngx.env credential file: written on save, pre-fills forms on load
- Dockerfile with layer-cached uv build, Python healthcheck
- docker-compose with host networking for Tailscale access
- Gitea Actions workflow: version bump, secret injection, docker compose deploy

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/metrics.py

@@ -0,0 +1,36 @@
+from prometheus_client import Counter, Gauge, Histogram
+
+docs_total = Counter(
+    "pngx_sync_docs_total",
+    "Total documents synced",
+    ["replica", "status"],
+)
+
+sync_duration = Histogram(
+    "pngx_sync_duration_seconds",
+    "Sync cycle duration in seconds",
+    ["triggered_by"],
+)
+
+replica_lag = Gauge(
+    "pngx_replica_lag_seconds",
+    "Seconds since last successful sync",
+    ["replica"],
+)
+
+replica_pending_tasks = Gauge(
+    "pngx_replica_pending_tasks",
+    "Pending tasks awaiting resolution",
+    ["replica"],
+)
+
+replica_consecutive_failures = Gauge(
+    "pngx_replica_consecutive_failures",
+    "Consecutive sync cycle failures per replica",
+    ["replica"],
+)
+
+sync_running = Gauge(
+    "pngx_sync_running",
+    "Whether a sync cycle is currently running",
+)