refactor: change OAuth scope separator from colon to dot for IDP compatibility

Many identity providers (AWS Cognito, Okta, Azure AD) reject or mishandle
colons in OAuth scope names. This migrates all custom scopes from
`resource:action` to `resource.action` format (e.g., `notes:read` →
`notes.read`), which is universally accepted and aligns with industry
conventions (Microsoft, Google).

Includes Alembic migration 004 for stored scope strings and ADR-024
documenting the rationale and RFC references.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/unit/test_scope_authorization_stored.py

@@ -29,7 +29,7 @@ async def test_get_stored_scopes_with_scopes():
     mock_storage = AsyncMock()
     mock_storage.get_app_password_with_scopes.return_value = {
         "app_password": "xxxxx",
-        "scopes": ["notes:read", "calendar:read"],
+        "scopes": ["notes.read", "calendar.read"],
         "username": "alice",
         "created_at": 1000,
         "updated_at": 1000,
@@ -41,7 +41,7 @@ async def test_get_stored_scopes_with_scopes():
     ):
         result = await _get_stored_scopes("alice")
 
-    assert result == ["notes:read", "calendar:read"]
+    assert result == ["notes.read", "calendar.read"]
 
 
 async def test_get_stored_scopes_null_scopes():