fix(api): serialize response datetimes as UTC

[justonemorenight]

What

Fixes #2174 by making REST datetime responses explicit UTC values.

SQLite drops tzinfo when round-tripping SQLAlchemy datetime columns, so API responses such as started_at arrived as suffix-less ISO strings. Browsers then interpreted them as local time, making elapsed job timers start at the viewer’s UTC offset (or stay at zero west of UTC).

Changes

• Add one shared UTCDateTime Pydantic type for REST response schemas.
• Serialize naive database values as UTC and aware values normalized to UTC, using an explicit Z suffix.
• Apply it to all 30 datetime response fields across repository, page/job, git, ownership, decision, code-quality, and chat schemas.
• Keep persistence unchanged; this is an API-boundary compatibility fix for existing SQLite and PostgreSQL data.

This covers the affected consumers called out in #2174: job elapsed/finished timestamps, repository timestamps, conversation age, dead-code timestamps, and git/ownership timestamps.

Tests

• uv run ruff check packages/server/src/repowise/server/schemas tests/unit/server/test_datetime_serialization.py
• uv run pytest tests/unit/server/test_datetime_serialization.py tests/unit/server/test_repos.py tests/unit/server/test_pages.py tests/unit/server/test_git.py tests/unit/server/test_chat_router.py tests/unit/server/test_decisions_api.py tests/unit/server/test_stats_contributors.py -q — 108 passed
• uv run pytest tests/unit/server --ignore=tests/unit/server/mcp -q — 1,391 passed

The full API contract remains backward-compatible for consumers that parse ISO-8601 timestamps; the only change is that UTC is now declared instead of being inferred as browser-local time.