docs: remediate documentation gaps across quickstart, backup, config, API spec, and operations guides (#330)

- Fix Cyrillic character in quickstart image ref and correct registry to Docker Hub (saelix/sencho)
- Correct backup guide WAL references (Sencho uses SQLite default journal mode)
- Add SSL/TLS reverse proxy examples for Nginx, Traefik, and new Caddy configuration
- Add missing env vars (PORT, DATA_DIR, NODE_ENV, FRONTEND_URL, SSO_LDAP_DISPLAY_NAME) to .env.example
- Add upgrade & migration guide documenting automatic schema migrations
- Add self-hosting best practices (1:1 path rule, Docker socket security, resource recs)
- Add architecture overview (system design, request flow, database schema, multi-node model)
- Add development & contributor guide (setup, tests, code style, PR workflow)
- Update OpenAPI spec from v0.23.0 to v0.25.3 with Registries and Image Updates endpoints
- Update docs.json navigation with all new pages and API groups

docs/getting-started/quickstart.mdx

@@ -18,7 +18,7 @@ docker run -d \
   -v /opt/compose:/app/compose \
   -v sencho_data:/app/data \
   -e JWT_SECRET=change-me \
-  ghcr.io/ansоcode/sencho:latest
+  saelix/sencho:latest
 ```
 
 Open `http://localhost:3000` in your browser. On first boot you'll be prompted to create an admin account.