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/server/auth_tools.py

@@ -56,7 +56,7 @@ def register_auth_tools(mcp: FastMCP) -> None:
 
         Args:
             ctx: MCP context
-            scopes: Requested application scopes (e.g. ["notes:read", "calendar:write"]).
+            scopes: Requested application scopes (e.g. ["notes.read", "calendar.write"]).
                     If not specified, all available scopes are requested.
 
         Returns: