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/config.py

@@ -0,0 +1,24 @@
+import os
+from functools import lru_cache
+
+
+class Config:
+    def __init__(self) -> None:
+        self.secret_key: str = os.environ.get("SECRET_KEY", "")
+        self.database_url: str = os.environ.get("DATABASE_URL", "sqlite:////data/db.sqlite3")
+        self.master_url: str | None = os.environ.get("MASTER_URL")
+        self.master_token: str | None = os.environ.get("MASTER_TOKEN")
+
+    @property
+    def db_path(self) -> str:
+        url = self.database_url
+        if url.startswith("sqlite:////"):
+            return "/" + url[len("sqlite:////"):]
+        if url.startswith("sqlite:///"):
+            return url[len("sqlite:///"):]
+        return url
+
+
+@lru_cache
+def get_config() -> Config:
+    return Config()