feat: Implement ADR-004 Hybrid Flow with comprehensive integration tests

Implement the ADR-004 Hybrid Flow OAuth pattern where the MCP server
intercepts the OAuth callback to obtain master refresh tokens while
maintaining PKCE security for clients.

## Implementation

### OAuth Routes (ADR-004 Hybrid Flow)
- Add `/oauth/authorize` endpoint: Intercepts client OAuth initiation
- Add `/oauth/callback` endpoint: Receives IdP callback, stores master token
- Add `/oauth/token` endpoint: Exchanges MCP code for client access token
- Implement PKCE code challenge/verifier validation
- Store OAuth sessions with state/challenge correlation

### MCP Server Integration
- Update `setup_oauth_config()` to return client_id and client_secret
- Initialize OAuth context in Starlette lifespan for login routes
- Add OAuth session storage to RefreshTokenStorage
- Configure authlib dependency for OAuth flow management

### Integration Tests
- Create `test_adr004_hybrid_flow.py` with Playwright automation
- Add `adr004_hybrid_flow_mcp_client` session-scoped fixture
- Test MCP session establishment with hybrid flow token
- Test tool execution using stored refresh tokens (on-behalf-of pattern)
- Test persistent access across multiple operations
- All tests passing: ✅ 3 passed in 8.82s

### Documentation
- Update ADR-004 with comprehensive Testing section
- Add integration test commands and coverage details
- Document test implementation and verification steps
- Create TESTING_INSTRUCTIONS.md for manual and automated testing
- Include manual test scripts for reference/debugging

## What This Enables

✅ PKCE code challenge/verifier flow
✅ MCP server intercepts OAuth callback and stores master refresh token
✅ Client receives MCP access token (not master token)
✅ MCP session establishment with hybrid flow token
✅ Tool execution using stored refresh tokens (on-behalf-of pattern)
✅ Multiple operations without re-authentication
✅ Proper token isolation (client never sees master token)

## Testing

Run ADR-004 integration tests:
```bash
uv run pytest tests/server/oauth/test_adr004_hybrid_flow.py --browser firefox -v
```

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

tests/manual/README.md

@@ -0,0 +1,47 @@
+# Manual OAuth Flow Testing
+
+This directory contains manual test scripts for OAuth flows that require browser interaction.
+
+## ADR-004 OAuth Hybrid Flow Test
+
+The `test_adr004_oauth_flow.py` script tests the complete OAuth flow described in ADR-004.
+
+### Prerequisites
+
+1. **Install Playwright browsers:**
+   ```bash
+   uv run playwright install firefox
+   ```
+
+2. **Start MCP server with OAuth enabled:**
+
+   For Nextcloud OIDC:
+   ```bash
+   export ENABLE_OFFLINE_ACCESS=true
+   export TOKEN_ENCRYPTION_KEY=$(uv run python -c "from cryptography.fernet import Fernet; print(Fernet.generate_key().decode())")
+   docker-compose up --build -d mcp-oauth
+   ```
+
+   For Keycloak:
+   ```bash
+   export ENABLE_OFFLINE_ACCESS=true
+   export TOKEN_ENCRYPTION_KEY=$(uv run python -c "from cryptography.fernet import Fernet; print(Fernet.generate_key().decode())")
+   docker-compose up --build -d mcp-keycloak
+   ```
+
+### Running the Test
+
+**Test with Nextcloud OIDC:**
+```bash
+uv run python tests/manual/test_adr004_oauth_flow.py --provider nextcloud
+```
+
+**Test with Keycloak:**
+```bash
+uv run python tests/manual/test_adr004_oauth_flow.py --provider keycloak
+```
+
+**Headless mode:**
+```bash
+uv run python tests/manual/test_adr004_oauth_flow.py --provider nextcloud --headless
+```