fix(auth): Store app passwords locally for multi-user BasicAuth background sync

Previously, the multi-user BasicAuth mode attempted to retrieve app passwords
via OAuth client_credentials grant, which Nextcloud OIDC doesn't support.

This fix implements local storage for app passwords:
- Add app_passwords table via Alembic migration (002)
- Add store/get/delete methods to RefreshTokenStorage
- Add management API endpoints for app password provisioning:
  - POST /api/v1/users/{user_id}/app-password
  - GET /api/v1/users/{user_id}/app-password
  - DELETE /api/v1/users/{user_id}/app-password
- Update oauth_sync.py to read from local storage
- Update Astrolabe to send app passwords to MCP server after validation
- Add app-hook to configure mcp_server_url in Nextcloud

The flow is now:
1. User creates app password in Nextcloud Security settings
2. User enters it in Astrolabe Personal Settings
3. Astrolabe validates against Nextcloud, then sends to MCP server
4. MCP server stores encrypted app password locally
5. Background sync uses locally stored password

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

nextcloud_mcp_server/alembic/versions/20260113_1200_002_add_app_passwords.py

@@ -0,0 +1,50 @@
+"""Add app_passwords table for multi-user BasicAuth mode
+
+This migration adds support for storing app passwords that are provisioned
+via Astrolabe's personal settings. This enables background sync in
+multi-user BasicAuth mode without requiring OAuth.
+
+Revision ID: 002
+Revises: 001
+Create Date: 2026-01-13 12:00:00.000000
+
+"""
+
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision = "002"
+down_revision = "001"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    """Add app_passwords table for multi-user BasicAuth mode."""
+
+    # App passwords table for multi-user BasicAuth background sync
+    op.execute(
+        """
+        CREATE TABLE IF NOT EXISTS app_passwords (
+            user_id TEXT PRIMARY KEY,
+            encrypted_password BLOB NOT NULL,
+            created_at INTEGER NOT NULL,
+            updated_at INTEGER NOT NULL
+        )
+        """
+    )
+
+    # Index for efficient user lookups
+    op.execute(
+        """
+        CREATE INDEX IF NOT EXISTS idx_app_passwords_updated
+        ON app_passwords(updated_at)
+        """
+    )
+
+
+def downgrade() -> None:
+    """Drop app_passwords table."""
+
+    op.execute("DROP INDEX IF EXISTS idx_app_passwords_updated")
+    op.execute("DROP TABLE IF EXISTS app_passwords")