feat(db): Add migration 010 for chunk config tracking columns

Add chunk_max_bytes and chunk_count columns to embedding_metadata to
support config drift detection and adaptive dedup sizing. Includes a
partial index on sentinel rows (chunk_index=0) to accelerate the drift
detection and max-chunk queries.

Also exports LATEST_SCHEMA_VERSION as a public constant derived from
the MIGRATIONS array length, replacing the previously hardcoded magic
number in the health check.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

migrations/010_chunk_config.sql

@@ -0,0 +1,14 @@
+-- Migration 010: Chunk config tracking + adaptive dedup support
+-- Schema version: 10
+
+ALTER TABLE embedding_metadata ADD COLUMN chunk_max_bytes INTEGER;
+ALTER TABLE embedding_metadata ADD COLUMN chunk_count INTEGER;
+
+-- Partial index: accelerates drift detection and adaptive dedup queries on sentinel rows
+CREATE INDEX idx_embedding_metadata_sentinel
+  ON embedding_metadata(document_id, chunk_index)
+  WHERE chunk_index = 0;
+
+INSERT INTO schema_version (version, applied_at, description)
+VALUES (10, strftime('%s', 'now') * 1000,
+  'Add chunk_max_bytes and chunk_count to embedding_metadata');