ci: add ruff linting and mypy type checking

Add ruff and mypy to dev dependencies with configuration in pyproject.toml.
Add a lint CI job that runs ruff check, ruff format --check, and mypy.
Auto-fix import sorting and formatting across the codebase. Exclude
alembic/versions from linting (auto-generated migrations). Ignore B008
(FastAPI Depends pattern) and RUF001 (unicode in user-facing strings).

21 ruff lint errors and 50 mypy errors remain for manual review.

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

tests/unit/services/domains/arxiv/test_guards.py

@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-from datetime import datetime, timezone
+from datetime import UTC, datetime
 
 from app.services.domains.arxiv.guards import arxiv_skip_reason_for_item
 from app.services.domains.publication_identifiers.types import DisplayIdentifier
@@ -25,7 +25,7 @@ def _item(
         pub_url=pub_url,
         pdf_url=pdf_url,
         is_read=False,
-        first_seen_at=datetime.now(timezone.utc),
+        first_seen_at=datetime.now(UTC),
         is_new_in_latest_run=True,
         display_identifier=display_identifier,
     )