feat: resolve GitHub webhook user_id — replace default_user with repo-mapped identity

[iamaryan07]

Summary

Problem

GitHub webhook events (PR merged, PR reviewed, commit pushed) were being
received by webhooks.py but never saved to the database. The commented-out
save code had "user_id": "default_user" hardcoded — there was no mechanism
to know which platform user the incoming webhook belonged to.

Solution

Added a reverse mapping (repo_db) in github_auth.py that stores
"owner/repo" → user_id whenever a user registers a webhook via /track-repo.
webhooks.py now reads repository.full_name from the incoming payload,
looks up the real user_id from that mapping, and saves the signal.

Changes

• github_auth.py — added repo_db: dict[str, str]; populated on
  /track-repo success (including 422 "already exists"); cleared on
  /disconnect-repo
• webhooks.py — un-commented and rewired the signal save block; imports
  repo_db, store, Signal; skips silently (with warning log) if the
  repo is not registered; saves a proper Signal object with the real
  user_id

Behaviour

• Signals from GitHub events are now saved with the correct user's UUID
• If a webhook fires for an unregistered repo, the event is dropped with
  a console warning (no crash, no phantom data)
• No changes to any other files — no merge conflicts expected

Known limitation

repo_db (and token_db) are in-memory. A server restart clears all
registered repos — users would need to re-register via /track-repo.
This is a pre-existing limitation of the architecture, not introduced here.