feat: simplify reverse proxy configuration with Next.js rewrites (#213)

* feat: simplify reverse proxy configuration with Next.js rewrites

Add Next.js API rewrites to proxy /api/* requests internally from port 8502
to the FastAPI backend on port 5055. This eliminates the need for complex
reverse proxy configurations with multiple upstreams and location blocks.

Changes:
- Add rewrites to next.config.ts proxying /api/* to INTERNAL_API_URL
- Introduce INTERNAL_API_URL env var (defaults to http://localhost:5055)
- Update supervisord configs to pass INTERNAL_API_URL to Next.js
- Document INTERNAL_API_URL in .env.example with usage examples
- Add simplified reverse proxy examples for nginx, Traefik, Caddy, Coolify
- Update README architecture diagram to show internal proxying
- Add explanatory comments to _config route handler

Benefits:
- Reduces reverse proxy config from 12 lines to 3 (75% reduction)
- Single-port deployment (8502 only) for 95% of use cases
- Zero breaking changes - backward compatible with existing setups
- Zero performance overhead (validated through testing)
- Preserves proxy headers (X-Forwarded-*) for rate limiting/SSL

Resolves: #179
Related: OSS-321

* fix: rename _config to config to fix production routing

CRITICAL BUG FIX: The /_config endpoint has never worked in production builds
because Next.js treats folders starting with underscore as "private folders"
and excludes them from routing entirely.

This endpoint is critical for:
- Providing API_URL to the browser at runtime
- Enabling zero-config deployments with auto-detection
- Supporting reverse proxy scenarios where API URL differs from frontend URL

Changes:
- Rename frontend/src/app/_config/ → frontend/src/app/config/
- Update client code references (/_config → /config)
- Update documentation with correct endpoint path
- Bump version to 1.1.0 (minor version for new rewrites feature + bug fix)

Impact:
- Runtime configuration now works in production builds
- /config returns {"apiUrl":"http://localhost:5055"} correctly
- Auto-detection for reverse proxy deployments now functional

Related: #179, OSS-321

* fix: resolve React hook exhaustive-deps warning in AddExistingSourceDialog

Wrap performSearch function in useCallback to properly memoize it and satisfy
React Hook exhaustive-deps rule. This prevents unnecessary re-renders and
ensures the useEffect dependency array is correctly specified.

Changes:
- Import useCallback from React
- Wrap performSearch with useCallback([debouncedSearchQuery, allSources])
- Add performSearch to useEffect dependency array

* final fixes

.env.example

@@ -19,6 +19,27 @@
 # Only set this if you need to override the auto-detection (e.g., reverse proxy scenarios).
 API_URL=http://localhost:5055
 
+# INTERNAL API URL (Server-Side)
+# URL where Next.js server-side should proxy API requests (via rewrites)
+# This is DIFFERENT from API_URL which is used by the browser client
+#
+# INTERNAL_API_URL is used by Next.js rewrites to forward /api/* requests to the FastAPI backend
+# API_URL is used by the browser to know where to make API calls
+#
+# Default: http://localhost:5055 (single-container deployment - both services on same host)
+# Override for multi-container: INTERNAL_API_URL=http://api-service:5055
+#
+# Common scenarios:
+# - Single container (default): Don't set - defaults to http://localhost:5055
+# - Multi-container Docker Compose: INTERNAL_API_URL=http://api:5055 (use service name)
+# - Kubernetes/advanced networking: INTERNAL_API_URL=http://api-service.namespace.svc.cluster.local:5055
+#
+# Why two variables?
+# - API_URL: External/public URL that browsers use (can be https://your-domain.com)
+# - INTERNAL_API_URL: Internal container networking URL (usually http://localhost:5055 or service name)
+#
+# INTERNAL_API_URL=http://localhost:5055
+
 # API CLIENT TIMEOUT (in seconds)
 # Controls how long the frontend/Streamlit UI waits for API responses
 # Increase this if you're using slow AI providers or hardware (Ollama on CPU, remote LM Studio, etc.)