fix(pgvector): make doc deletion query faster and use chunking

[kyteinsky]

CI logging for slow queries has also been enabled, not sure if we will see that in the CI though.

Sample output for the slow deletion query where a missing index on the source_id foreign key in access_list table was the culprit.
Calculated time: 3.495 + 0.310 + 0.129 = 3.934 ms
Actual time: 201177.123 ms or 201 s

        Query Text: DELETE FROM docs WHERE docs.source_id IN ($1::VARCHAR, $2::VARCHAR, ..., $275::VARCHAR) RETURNING docs.chunks
        Query Parameters: ...
        Delete on docs  (cost=1126.32..2018.25 rows=275 width=6) (actual time=0.192..3.495 rows=218 loops=1)
    ->  Bitmap Heap Scan on docs  (cost=1126.32..2018.25 rows=275 width=6) (actual time=0.144..0.310 rows=218 loops=1)
                Recheck Cond: ((source_id)::text = ANY ('{"files__default: 20392","files__default: 23092", ... }'::text[]))
                Heap Blocks: exact=25
                ->  Bitmap Index Scan on docs_pkey  (cost=0.00..1125.56 rows=275 width=0) (actual time=0.129..0.129 rows=218 loops=1)
                      Index Cond: ((source_id)::text = ANY ('{"files__default: 20392", ...
2026-03-19 11:28:59.760 UTC [6703] LOG:  duration: 201177.123 ms  execute <unnamed>: DELETE FROM docs WHERE docs.source_id IN ($1::VARCHAR, $2::VARCHAR, ..., $275::VARCHAR) RETURNING docs.chunks
2026-03-19 11:28:59.760 UTC [6703] DETAIL:  Parameters: $1 = 'files__default: 20392', $2 = ...

(put the chunking part in a different PR)

[kyteinsky]

DB migration needs to be done for this to happen on existing installations.