[BUG] Cron backfill silently swallows database errors

[realproject7]

Problem

PR #54 ([#12] Cron Backfill) merged with silent error handling on all database upsert operations. The inline indexers (plot, storyline, donation) correctly check { error } from Supabase upserts and return appropriate HTTP errors — the backfill does not.

Issues

1. No error check on upsert operations

All three event processors (processStorylineCreated, processPlotChained, processDonation) ignore the return value of supabase.from(...).upsert(...). If an insert fails (FK violation, constraint error, etc.), the backfill silently continues and reports success.

Compare inline indexer (correct):

const { error: dbError } = await supabase.from("storylines").upsert(...);
if (dbError) {
  return error(`Database error (storyline): ${dbError.message}`, 500);
}

Backfill (missing):

await supabase.from("storylines").upsert(storylineRow, { onConflict: "tx_hash,log_index" });
// ← No error handling

2. Orphaned genesis plot risk

In processStorylineCreated, the genesis plot upsert runs immediately after the storyline upsert. If the storyline insert fails, the plot insert also fails at the FK constraint level — but neither error is reported.

3. Error counter only tracks decode failures

The errors counter in the main handler only increments on catch (decoding/IPFS failures). DB failures are not counted, so the response { processed, errors } is misleading.

4. Cursor update not checked

The final backfill_cursor update also doesn't check for errors.

Acceptance Criteria

• All upsert() calls destructure { error } and log/count failures
• errors response field reflects both decode AND database failures
• Cursor update checks for errors
• Remove 3 unused imports (plotChainedEvent, storylineCreatedEvent, donationEvent)
• npm run lint and npm run typecheck pass