indexer: fetch retry/backoff, loud placeholder warning, streams + stream_events folds (#566–#569)

[markdavid000]

Summary

Fixes four indexer scaffold gaps that together meant the worker could either
stall invisibly or never index the protocol it ships alongside.

closes #569 — no retry/backoff around the fetch step

• Adds indexer/src/retry.ts — retryWithBackoff(): exponential (default 3
  attempts, 250ms base doubling to a 5s cap), abort-signal aware, emits one
  structured warn line per retry so a retrying worker never looks "healthy
  but idle".
• The RPC fetch in pollOnce() and Poller.tick() now runs through it.
  The cursor is only checkpointed after a successful fetch; when retries are
  exhausted the error is logged, the cursor stays put, and the next tick /
  loop iteration resumes from the same position.
• Ingest failures stay transactional (BEGIN → ingest → saveCursor → COMMIT,
  rollback on failure) and are retried by the loop-level backoff in
  startPollLoop — now documented in code rather than incidental.

closes #568 — stub event source silently indexed nothing

• StubSorobanEventSource now logs a one-time warning on construction:
  "using placeholder SorobanEventSource — no events will be indexed".
• src/worker.ts warns once at boot and on first fetch, covering the
  npm run start:worker path where the placeholder lives in
  fetchEventsFromRPC. "Intentionally running the scaffold" and "the real
  RPC implementation is broken and returning nothing" are now distinguishable
  from the logs.

closes #567 — no table tracks stream creation

• New streams table: id, sender, recipient, token, rate_per_second, start_time, end_time, created_at.
• New handleStreamCreated fold in applyEvent (handles created and
  stream_created tags) with an idempotent ON CONFLICT (id) DO UPDATE —
  created_at is never rewritten, so "streams created in the last 24h"
  survives page replays.

closes #566 — applyEvent only handled DAO-voting events

• New stream_events table (PK ledger, tx_hash, event_type, stream_id).
• applyEvent now folds stream_withdrawn and stream_cancelled (plus
  force_cxl, stream_paused, stream_resumed, stream_topped_up,
  stream_clawback, xfer_rec) using the existing upsert pattern, accepting
  both the README event names and the raw on-chain topic aliases.
• Unrecognized types still log-and-drop (indexer: applyEvent's default: return silently drops unrecognized event types #578).

Acceptance criteria

• indexer: poller.ts has no retry/backoff around fetchPage/ingestPage failures #569 explicit retry-with-backoff around the RPC fetch step
• indexer: poller.ts has no retry/backoff around fetchPage/ingestPage failures #569 test asserting the poller doesn't silently stop advancing after a
  single transient failure (both pollers)
• indexer: chainEventSource.ts stub silently returns an empty page forever #568 one-time warning on stub construction
• indexer: no derived table tracks DripFactory::create_stream (stream creation) #567 streams table with the required columns + stream-creation fold
• indexer: applyEvent has no handler for any DripStream event — only DAO-voting events #566 stream_events table + handlers for at least stream_withdrawn
  and stream_cancelled, following the existing upsert pattern

Testing

• 33 new tests across retry, poller, indexerPoller, applyEvent,
  placeholderSource, schema
• npm run lint / npm run typecheck / npm test (indexer CI steps): 72
  passed, 1 skipped, 0 failed
• Drive-by: fixed the pre-existing prefer-const lint error in worker.ts
  that was keeping the indexer CI job red