feat: add CSV export endpoint for alerts (#521)

[Acruve15]

Summary

• Adds GET /alerts/export?from_date=YYYY-MM-DD&to_date=YYYY-MM-DD returning a CSV streamed download with columns id, lat, lon, started_at, last_seen_at.
• Filters alerts by started_at within the inclusive UTC window [from_date 00:00:00, to_date 23:59:59.999999] and scopes results to the caller's organization_id.
• Implements the first iteration of #521; the issue's "Future improvements" (sequence ids, camera names) are deliberately left for a follow-up.

Implementation notes

• Route declared before /{alert_id} in alerts.py so FastAPI doesn't try to parse export as an int path param.
• Uses stdlib csv + io.StringIO + StreamingResponse (no new dependencies; pandas not needed for this shape).
• Inline select(Alert).where(...) mirrors fetch_alerts_from_date; nothing added to the CRUD layer.
• DB columns store naive UTC datetimes (per app.core.time.utcnow), so the bounds are built without tzinfo.
• Returns 422 when to_date < from_date, 401 when unauthenticated, 200 with header-only body for an empty range.

Test plan

• pytest tests/endpoints/test_alerts.py -v (all 10 tests, including 6 new export tests, pass)
  • Happy path + ascending order
  • Window narrows correctly (only middle-day alert returned)
  • Org isolation (org 2 alerts absent when called as org 1 user)
  • Empty range returns header-only CSV
  • to_date < from_date → 422
  • No token → 401
• ruff 0.11.9 check and ruff 0.11.9 format --check pass
• Manually curl the new endpoint with a valid token and confirm the CSV opens cleanly

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.92%. Comparing base (1f67522) to head (656df3e).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #577      +/-   ##
==========================================
+ Coverage   88.75%   89.92%   +1.17%     
==========================================
  Files          52       55       +3     
  Lines        2196     2451     +255     
==========================================
+ Hits         1949     2204     +255     
  Misses        247      247              

Flag	Coverage Δ
backend	89.89% <100.00%> (+1.24%)	⬆️
client	90.40% <ø> (ø)

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[fe51]

Hi Alexis, thanks a lot for this PR and sorry for the review delay !

Tested it works, and reviewed the code !
This is clean, and quite efficient (have not tested to extract for long time period but the content is light)

I have juste added one remark/question in tests but non blocking to me !