add job phase timing logs

[reobin]

the worker currently logs only coarse markers and final elapsed time, which made it hard to identify where import and update were spending time.

• log phase durations for import: github search, data preparation, repository upsert, job event writes
• log phase durations for update: repository load, github fetches, database writes, deletes/errors
• keep logs concise enough for cloudwatch scanning
• include counts per phase where useful