Report weekly dataset/model counts to services reporting webhook

[nee1k]

Summary

• Adds an opt-in weekly background job (ENABLE_WEEKLY_REPORT) that POSTs Patra's total model-card and datasheet counts to an external webhook (Carlos's services reporting board), per Beth's request — the board is currently being showcased.
• Plain asyncio background task started/cancelled from rest_server/main.py's existing lifespan hook — no new dependency. Reports once immediately on startup, then every 7 days.
• Webhook URL, auth token, and JSON payload shape (WEEKLY_REPORT_WEBHOOK_URL / WEEKLY_REPORT_WEBHOOK_TOKEN) are placeholders — Carlos's real spec isn't available yet. build_payload()/post_report() in rest_server/features/weekly_report/service.py are isolated so updating them later is a small, contained change.
• Off by default everywhere (including docker-compose.backend.yml); a no-op even when enabled if the webhook URL isn't set.

Test plan

• pytest tests/test_weekly_report.py -v — 13/13 new tests pass (count query, payload shape, webhook POST success/failure/auth, flag-gating, lifespan wiring)
• pytest tests/ -v --ignore=tests/test_mcp_server.py — full suite, 200/200 pass, no regressions
• Manual smoke test: built and ran the real Docker stack with ENABLE_WEEKLY_REPORT=true pointed at a local listener — confirmed an immediate POST on startup with the expected payload shape and Authorization: Bearer <token> header
• Confirm Carlos's actual webhook URL/auth/payload spec and update build_payload()/post_report() accordingly before enabling in production