fix: handle 'skipped' status in job_detail_status updates

- Add 'skipped' to terminal states in update_job_detail_status()
- Ensures skipped dates properly:
  - Update status and completed_at timestamp
  - Store skip reason in error field
  - Trigger job completion checks
- Add comprehensive test suite (11 tests) covering:
  - Database schema validation
  - Job completion with skipped dates
  - Progress tracking with skip counts
  - Multi-model skip handling
  - Skip reason storage

Bug was discovered via TDD - created tests first, which revealed
that skipped status wasn't being handled in the terminal state
block at line 397.

All 11 tests passing.

api/job_manager.py

@@ -394,7 +394,7 @@ class JobManager:
                     WHERE job_id = ? AND status = 'pending'
                 """, (updated_at, updated_at, job_id))
 
-            elif status in ("completed", "failed"):
+            elif status in ("completed", "failed", "skipped"):
                 # Calculate duration for detail
                 cursor.execute("""
                     SELECT started_at FROM job_details