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/routers/tasks.py

@@ -8,6 +8,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 from api.database import get_session
 from api.models.task import Task, TaskStatus
 from api.models.token_event import TokenEvent
+from api.models.workstream import Workstream
 from api.schemas.task import TaskCreate, TaskRead, TaskUpdate
 
 router = APIRouter(prefix="/tasks", tags=["tasks"])
@@ -104,9 +105,14 @@ async def update_task(
             # Tier 3: heuristic fallback
             tin, tout, tnote = 1000, 500, "heuristic"
 
+        # Resolve repo_id via workstream
+        ws = await session.get(Workstream, task.workstream_id)
+        repo_id = ws.repo_id if ws else None
+
         event = TokenEvent(
             task_id=task_id,
             workstream_id=task.workstream_id,
+            repo_id=repo_id,
             tokens_in=tin,
             tokens_out=tout,
             model=token_data.get("model"),