refactor: enforce PLC0415 (import-outside-top-level) for source code

Enable ruff PLC0415 rule for all source files (tests excluded via
per-file-ignores). Move 136 inline imports to top-level across 33 files.
8 imports suppressed with noqa for legitimate reasons: circular
dependencies (client/__init__.py, context.py), optional dependency
guards (app.py document processors, auth/userinfo_routes.py), and
post-env-setup imports (smithery_main.py).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

nextcloud_mcp_server/config.py

@@ -268,9 +268,7 @@ class Settings:
                 "This is insecure and should only be used for development/testing."
             )
         if self.nextcloud_ca_bundle:
-            import os as _os
-
-            if not _os.path.isfile(self.nextcloud_ca_bundle):
+            if not os.path.isfile(self.nextcloud_ca_bundle):
                 raise ValueError(
                     f"NEXTCLOUD_CA_BUNDLE path does not exist: {self.nextcloud_ca_bundle}"
                 )