chore(backend): Ruff lint pass — deprecated APIs, exception leaks, dead patterns

Mechanical sweep of items called out in the PR review:

- qwen_llm_backend: AutoModelForCausalLM.from_pretrained(torch_dtype=…) is deprecated in transformers ≥4.41 in favor of dtype=. Renamed.
- routes/llm: try/except around backend.generate() raised HTTPException(500, detail=str(e)) which leaks stack traces / paths to clients and trips Ruff B904. Now logs the original exception server-side and hands the client a generic message; chained via `from e` to preserve traceback context.
- mcp_bindings + mcp_server/context: datetime.utcnow() is deprecated since 3.12. Switched the two assignment sites to datetime.now(timezone.utc). The schema-level `default=datetime.utcnow` defaults in database/models.py are left for a later schema-aware pass.
- routes/generations: `logger = …` sat between two import blocks (Ruff E402). Moved below imports.
- mcp_server/server + tests/test_refinement_samples: typing.Callable / typing.Iterable have been preferred-via collections.abc since 3.9 (Ruff UP035).
- routes/events: `except asyncio.TimeoutError` aliases plain `TimeoutError` since 3.11 (UP041).
- services/captures: hoisted WHISPER_NATIVE_FORMATS to module scope (was a function-local UPPER_SNAKE that tripped N806) and replaced the raw_path.unlink try/except OSError-pass with contextlib.suppress (SIM105). Semantic equivalence preserved — written_files.remove(raw_path) still only runs when unlink succeeds because it sits inside the suppressed block after the unlink call.
- database/migrations: hoisted the duplicate `import sqlite3` from inside two helper bodies to a single module-level import.

backend/mcp_server/context.py

@@ -11,7 +11,7 @@ import asyncio
 import ipaddress
 import logging
 from contextvars import ContextVar
-from datetime import datetime
+from datetime import datetime, timezone
 
 from starlette.middleware.base import BaseHTTPMiddleware
 from starlette.requests import Request
@@ -141,7 +141,7 @@ def _stamp_last_seen(client_id: str) -> None:
         if row is None:
             row = MCPClientBinding(client_id=client_id)
             db.add(row)
-        row.last_seen_at = datetime.utcnow()
+        row.last_seen_at = datetime.now(timezone.utc)
         db.commit()
     except Exception:
         logger.debug(