docs: comprehensive documentation overhaul

Add complete documentation structure:
- Frontend architecture documentation
- Database schema documentation
- Deployment guides (Docker, Traefik, Authentik, Environment)
- Development guides (Setup, Testing, Contributing, Quality Gates)
- Deployment architecture documentation
- Updated docs README with complete navigation

All new features and APIs are now documented.
Quality gates: docs only, no code changes

docs/api/auth.md

@@ -0,0 +1,105 @@
+# Auth API
+
+Authentication endpoints for OAuth2 login via Authentik.
+
+## Authentication
+
+These endpoints handle the OAuth2 flow. No prior authentication is required for `/auth/login` and `/auth/callback`.
+
+---
+
+## GET /auth/login
+
+**Description:** Initiate OAuth2 login flow. Redirects to Authentik.
+
+### Request
+
+#### Query Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|----------|-------------|
+| `next` | `string` | No | URL to redirect to after login |
+
+### Response
+
+#### Success (307 Temporary Redirect)
+
+Redirects to Authentik OAuth2 authorization URL.
+
+---
+
+## GET /auth/callback
+
+**Description:** Handle OAuth2 callback from Authentik.
+
+### Request
+
+#### Query Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|----------|-------------|
+| `code` | `string` | Yes | Authorization code from Authentik |
+| `state` | `string` | Yes | State parameter for CSRF protection |
+
+### Response
+
+#### Success (307 Temporary Redirect)
+
+Sets session cookie and redirects to frontend.
+
+#### Error (400 Bad Request)
+
+```json
+{
+  "detail": "Invalid state parameter"
+}
+```
+
+---
+
+## GET /auth/me
+
+**Description:** Get current authenticated user.
+
+**Auth:** Required (session cookie)
+
+### Response
+
+#### Success (200 OK)
+
+```json
+{
+  "id": "uuid",
+  "email": "user@example.com",
+  "name": "User Name",
+  "avatar_url": "https://..."
+}
+```
+
+#### Error (401 Unauthorized)
+
+```json
+{
+  "detail": "Not authenticated"
+}
+```
+
+---
+
+## POST /auth/logout
+
+**Description:** Log out current user.
+
+**Auth:** Required (session cookie)
+
+### Response
+
+#### Success (200 OK)
+
+Clears session cookie.
+
+```json
+{
+  "message": "Logged out"
+}
+```