fix: return empty election lists with HTTP 200

[Gambit-Checkmate]

Summary

Closes #189.

• Return HTTP 200 with [] when GET /election finds no elections, both with and without with_nominees=true.
• Remove the collection endpoint's obsolete 404 response from OpenAPI and describe its empty-list response.
• Add HTTP-level unit tests for the default/false/true nominee query, the OpenAPI responses, and the unchanged 404 for a missing individual election.

Both CRUD list functions already return sequences, so removing the empty-result exceptions lets the existing serialization produce an empty array. Database queries, authorization, and individual-election lookup behavior are unchanged.

Verification

Windows, Python 3.13.15, locked development/test dependencies. The local environment additionally uses tzdata because Windows does not provide the system timezone database; no dependency or lockfile changes are included.

• Before the fix: the new tests produced 4 failures (the three empty collection requests and its OpenAPI response) and 1 pass (missing individual election).
• python -m pytest tests/unit -q: 61 passed (existing baseline: 56 passed).
• ruff check .: passed.
• ruff format --check src/elections/urls.py tests/unit/test_elections.py: passed.
• git diff --check: passed.

The new tests exercise the FastAPI router through HTTPX's ASGI transport with mocked database dependencies. PostgreSQL-backed integration tests were not run; no real database was connected or modified.

Opened as a draft following the project's PR checklist. Prepared with AI assistance.