* fix semantic search reindex
sqlite-vec added the `_info` shadow table in 0.1.6 and drops it unconditionally when a vec0 table is destroyed, so `DROP TABLE` on a table written by 0.17 failed with "SQL logic error" once 0.18 moved to 0.1.9. `SqliteQueueDatabase` queues non-SELECT statements and stores the exception on the cursor it returns, and nothing read those cursors, so the failed drop and every write after it went unreported while reindex still logged "Embedded N thumbnails". `drop_embeddings_tables()` now recreates the missing `_info` stub before dropping, and writes go through `execute_write()`, which waits on the cursor so failures raise. `INSERT OR REPLACE` is gone too, since vec0 implements neither REPLACE nor UPSERT and it always failed on an id already in the table, including under the 0.1.3 build 0.17 shipped.
* use lock
* show reindex failure in status bar