fix: resolve correct block by CID in firehose commit indexing
The indexer scanned all blocks for the first matching $type, so
multi-record commits (e.g. applyWrites with multiple photos) indexed
every op with the same first block's data. Look up each op's specific
block by its CID instead.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>