feat(signals): implement dynamic github event ingestion and goal_id

[Dynamic-ctrl]

Summary

Implements dynamic GitHub event polling to ingest real-time repository activity and strictly maps it to specific goals, fulfilling the missing "goal-to-signal syncing" pipeline requirement.

• Models: Updates SignalCreate and Signal in src/lpi/models.py to accept an optional goal_id, mapping perfectly to the new Supabase schema.
• API Route: Creates POST /api/v1/signals/sync-github/{goal_id} in src/lpi/routers/signals.py. Uses httpx.AsyncClient to non-blockingly poll the GitHub REST API (/events), filtering strictly for high-value PushEvent and PullRequestEvent payloads.
• Integration: Reuses store.insert_signal and log_user_activity to ensure immediate compliance with the new Row Level Security (RLS) policies requiring goal ownership validation.
• Impact: Unblocks Jaivardhan (LangGraph AI ingestion) and Jahanvi (Frontend UI timeline mapping).

Type

• Feature (new endpoint / model)
• Fix (bug, test failure)
• Refactor
• Docs / reports

Checklist

• make lint exits 0
• make test exits 0 (no new failures)
• Tests added or updated for changed behaviour
• Daily report updated in reports/
• No .env secrets committed
• PR targets staging (never main directly)

Tests passing

Daily report reference