Fix UNIQUE constraint crash when same --attachment passed twice

[abhicris]

Closes #1354.

The bug

$ llm "Describe this" --attachment resume.pdf --attachment resume.pdf
...
File ".../llm/models.py", line 924, in log_to_db
    db["prompt_attachments"].insert(...)
sqlite3.IntegrityError: UNIQUE constraint failed:
  prompt_attachments.response_id, prompt_attachments.attachment_id

The prompt_attachments table was created in m012 with a composite PK of (response_id, attachment_id), so inserting the same attachment a second time against one response violates the constraint. The full prompt + model call runs fine; only the log step crashes, which means the user sees a traceback instead of output.

The fix

This is the same bug that previously existed on prompt_fragments, fixed in m016_fragments_table_pks by extending the PK with the per-row order column. This PR mirrors that fix with a new migration:

@migration
def m022_prompt_attachments_pk(db):
    db["prompt_attachments"].transform(
        pk=("response_id", "attachment_id", "order")
    )

log_to_db() already passes a distinct order per attachment (enumerate(self.prompt.attachments)), so the logic side needs no change.

Verification

• Reproduced the IntegrityError against main at 1bfb96d.
• After the migration, both fresh databases and databases upgraded from pre-m022 schema accept duplicate (response_id, attachment_id) rows with distinct order.
• Added test_duplicate_attachment_logs_cleanly in tests/test_attachments.py exercising the CLI path end-to-end.
• Full test suite green (457 passed) excluding the existing openai-models + async-recording tests that require network/cassettes.

--- kcolbchain / Abhishek Krishna