feat(token-tracking): repo aggregation via graph walk (task→workstream→repo)

By Repo now resolves via the full chain rather than requiring repo_id
directly on the token event:
  1. token_events.repo_id (direct)
  2. → workstreams.repo_id (via workstream_id)
  3. → task.workstream_id → workstreams.repo_id (via task_id)

Changes:
- Auto-populate repo_id on token events at creation time (both the
  token_events router and the tasks router)
- New GET /token-events/by-repo/ endpoint with RepoTokenSummary schema;
  returns tokens_in/out/total, event_count, by_model, by_note per repo
- Dashboard By Repo section uses /by-repo/ directly and shows repo_slug
  instead of a truncated UUID
- Backfilled the three existing events (userbased) with repo_id via SQL

185 tests pass.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

api/schemas/token_event.py

@@ -50,3 +50,14 @@ class TokenSummary(BaseModel):
     event_count: int
     by_model: dict[str, int]
     by_agent: dict[str, int]
+
+
+class RepoTokenSummary(BaseModel):
+    repo_id: uuid.UUID
+    repo_slug: str
+    tokens_in: int
+    tokens_out: int
+    tokens_total: int
+    event_count: int
+    by_model: dict[str, int]
+    by_note: dict[str, int]