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/observability/metrics.py

@@ -25,6 +25,8 @@ from prometheus_client import (
     start_http_server,
 )
 
+from nextcloud_mcp_server.observability.tracing import trace_operation
+
 logger = logging.getLogger(__name__)
 
 # =============================================================================
@@ -426,8 +428,6 @@ def instrument_tool(func):
         Wrapped function with metrics and tracing instrumentation
     """
 
-    from nextcloud_mcp_server.observability.tracing import trace_operation
-
     @functools.wraps(func)
     async def wrapper(*args, **kwargs):
         tool_name = func.__name__