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>

nextcloud_mcp_server/observability/metrics.py

@@ -410,7 +410,7 @@ def instrument_tool(func):
 
     Usage:
         @mcp.tool()
-        @require_scopes("notes:write")
+        @require_scopes("notes.write")
         @instrument_tool
         async def nc_notes_create_note(...):
             ...