feat: add Alembic database migration system

Implements Alembic for managing token storage database schema versions.
Migrations run automatically on startup with full backward compatibility.

**Changes:**
- Add Alembic dependency (1.14.0+) and SQLAlchemy (auto-installed)
- Create migration infrastructure in alembic/ directory
- Add initial migration (001) capturing current schema
- Modify RefreshTokenStorage.initialize() to run migrations via anyio
- Add CLI commands: db upgrade, current, history, downgrade, migrate
- Add comprehensive migration documentation

**Backward Compatibility:**
- Pre-Alembic databases automatically stamped with revision 001
- No schema changes for existing databases
- Automatic upgrade on first startup after update

**Migration Strategy:**
Three scenarios handled:
1. New database → Run migrations from scratch
2. Pre-Alembic database → Stamp with 001 (no changes)
3. Alembic-managed → Upgrade to latest

**Architecture:**
- Uses anyio.to_thread.run_sync() for structured concurrency
- Alembic env.py runs with anyio.run() in worker thread
- SQLite-friendly migration patterns documented
- No ThreadPoolExecutor needed (anyio handles it)

**CLI Usage:**
```bash
nextcloud-mcp-server db upgrade    # Upgrade to latest
nextcloud-mcp-server db current    # Show version
nextcloud-mcp-server db history    # View changelog
nextcloud-mcp-server db downgrade  # Rollback (with confirmation)
nextcloud-mcp-server db migrate "description"  # Create migration
```

**Testing:**
- All 13 webhook storage tests pass
- New/pre-Alembic database scenarios validated
- anyio integration tested

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

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

nextcloud_mcp_server/auth/oauth_routes.py

@@ -517,12 +517,23 @@ async def oauth_callback_nextcloud(request: Request):
             token_data.get("scope", "").split() if token_data.get("scope") else None
         )
 
+        # Calculate refresh token expiration from token response
+        refresh_expires_in = token_data.get("refresh_expires_in")
+        refresh_expires_at = None
+        if refresh_expires_in:
+            import time
+
+            refresh_expires_at = int(time.time()) + refresh_expires_in
+            logger.info(f"  refresh_expires_in: {refresh_expires_in}s")
+            logger.info(f"  refresh_expires_at: {refresh_expires_at}")
+
         logger.info("Storing refresh token:")
         logger.info(f"  user_id: {user_id}")
         logger.info("  flow_type: flow2")
         logger.info("  token_audience: nextcloud")
         logger.info(f"  provisioning_client_id: {state[:16]}...")
         logger.info(f"  scopes: {granted_scopes}")
+        logger.info(f"  expires_at: {refresh_expires_at}")
 
         await storage.store_refresh_token(
             user_id=user_id,
@@ -531,7 +542,7 @@ async def oauth_callback_nextcloud(request: Request):
             token_audience="nextcloud",
             provisioning_client_id=state,  # Store which client initiated provisioning
             scopes=granted_scopes,
-            expires_at=None,  # Refresh tokens typically don't expire
+            expires_at=refresh_expires_at,
         )
         logger.info(f"✓ Stored Flow 2 master refresh token for user {user_id}")
         logger.info("=" * 60)