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/database/migrations.py

@@ -18,6 +18,7 @@ Adding a new migration:
 """
 
 import logging
+import sqlite3
 
 from sqlalchemy import inspect, text
 
@@ -266,8 +267,6 @@ def _migrate_mcp_bindings(engine, inspector, tables: set[str]) -> None:
             # Leaving the unused column in place is harmless — the ORM
             # only maps declared columns, so a stray one does no work
             # and gets no reads or writes.
-            import sqlite3
-
             logger.warning(
                 "SQLite %s too old to DROP COLUMN (need 3.35+); leaving unused default_intent column on mcp_client_bindings in place.",
                 sqlite3.sqlite_version,
@@ -280,8 +279,6 @@ def _supports_drop_column(engine) -> bool:
     decades; SQLite only gained the feature in 3.35."""
     if engine.dialect.name != "sqlite":
         return True
-    import sqlite3
-
     return tuple(int(p) for p in sqlite3.sqlite_version.split(".")[:3]) >= (3, 35, 0)