Add has_transcript cleanup command

[biast12]

Description

Some tickets have has_transcript = true but no transcript in S3, e.g. guilds that imported v1 ticket data without the transcripts. The dashboard shows a "View" button for them that fails.

• New cmd/fix-has-transcript: a one-off command that sets has_transcript = false for closed tickets whose transcript is missing from every bucket.
  • Checks all guilds by default. -guildid, -ticket, -csv and -after/-before narrow the scope; -dry-run only reports.
  • Prints each changed guild_id,ticket_id to stdout, which doubles as an undo list.
  • Uses logarchiver's DATABASE_URI/S3_* env; the main DB is passed with -dburi.
  • Safety: lists S3 twice before unsetting, skips tickets closed after the run started, aborts if recent transcripts can't be found (wrong env), and skips a guild on any error.
• GetTicket: a missing key now returns ErrTicketNotFound, so GET / responds 404 instead of 500 and the dashboard shows "Transcript not found".
• Upload handler: now responds 500 when the existing-object lookup fails. Before, it returned 200 without storing anything, so the worker set has_transcript = true for a transcript that was never saved.

Type of Change

• Bug fix
• New feature
• Breaking change

Testing

• Run the command end-to-end against local Postgres + MinIO:
  • open and reopened tickets
  • dry run, apply, -ticket, and a -csv round trip
• Compared the server before/after: missing transcript 500 → 404, failed objects lookup 200 → 500, normal upload and GET unchanged.

Checklist

• My code follows the style of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works