Complements the #65637 salvage (53d358838 + a9cc17fd8): the gateway
session store now retries transcript appends through its own queue, but
cron and CLI writers call SessionDB directly — a corrupt FTS index still
hard-failed their appends until the next process restart triggered the
offline repair.
_execute_write now detects the FTS-corruption error class (both the
generic 'database disk image is malformed' and newer SQLite's
'fts5: corrupt structure record' variant), performs a one-shot in-place
rebuild by delegating to the existing rebuild_fts(), and retries the
failed write. One-shot per instance so an unrecoverable database cannot
loop; lock/busy jitter-retry path untouched.
E2E-verified: corrupted messages_fts_data rejects appends; with this fix
the same append self-heals, persists, and FTS search works again.
9e1b1d7536