refactor: simplify path validation and connection_init via shared helpers

- Extract _validate_path() in unraid.py — consolidates traversal check + normpath
  + prefix validation used by disk/logs and live/log_tail into one place
- Extract build_connection_init() in subscriptions/utils.py — removes 4 duplicate
  connection_init payload blocks from snapshot.py (×2), manager.py, diagnostics.py;
  also fixes diagnostics.py bug where x-api-key: None was sent when no key configured
- Remove _LIVE_ALLOWED_LOG_PREFIXES alias — direct reference to _ALLOWED_LOG_PREFIXES
- Move import hmac to module level in server.py (was inside verify_token hot path)

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

unraid_mcp/subscriptions/manager.py

@@ -19,7 +19,7 @@ from ..config import settings as _settings
 from ..config.logging import logger
 from ..core.client import redact_sensitive
 from ..core.types import SubscriptionData
-from .utils import build_ws_ssl_context, build_ws_url
+from .utils import build_connection_init, build_ws_ssl_context, build_ws_url
 
 
 # Resource data size limits to prevent unbounded memory growth
@@ -284,13 +284,9 @@ class SubscriptionManager:
                     logger.debug(
                         f"[PROTOCOL:{subscription_name}] Initializing GraphQL-WS protocol..."
                     )
-                    init_type = "connection_init"
-                    init_payload: dict[str, Any] = {"type": init_type}
-
-                    if _settings.UNRAID_API_KEY:
+                    init_payload = build_connection_init()
+                    if "payload" in init_payload:
                         logger.debug(f"[AUTH:{subscription_name}] Adding authentication payload")
-                        # Use graphql-ws connectionParams format (direct key, not nested headers)
-                        init_payload["payload"] = {"x-api-key": _settings.UNRAID_API_KEY}
                     else:
                         logger.warning(
                             f"[AUTH:{subscription_name}] No API key available for authentication"