feat(handler): Phase 3E — Stage handlers (fetch/render/upload) + Job/Handler refactor

[smallchungus]

Phase 3E: real Gmail->PDF->Drive pipeline.

• Job model gains user_id, gmail_message_id, is_synthetic columns (read/written by Go now; schema columns existed since 3A).
• New store.AdvanceJob(jobID, nextStage) transitions a running job to next stage queued.
• Handler interface returns (nextStage, err). Worker.ProcessOne advances or marks done accordingly.
• Three stage handlers in internal/handler:
  • FetchHandler: gmail.FetchMessage -> /data/mime/.eml -> 'render'
  • RenderHandler: parse MIME, build HTML wrapper, gotenberg -> /data/pdf/.pdf -> 'upload'
  • UploadHandler: drive.EnsureFolder for YYYY/MM/DD (cached in Redis), drive.Upload PDF -> '' (terminal)
• cmd/worker switches on --stage to pick the handler. --stage=test keeps NoopHandler so loadtest still works without OAuth credentials.

Out of scope (Phase 3F): scheduler binary, manual OAuth setup helper, attachment preservation.