perf(post-content): bound the ordered backfill scan

[seonghobae]

Summary

• preserve post-content candidate semantics while expressing one-to-zero/one checks as correlated existence predicates
• add a replay-safe partial expression index matching the governed event-time ordering
• document the bounded scan contract in ADR 0098

Canonical PostgreSQL 16 evidence

• feat(orchestrator): scope endpoint routing selector #781 query before: 2526.445 ms, 462185 shared hits, 17666 reads, 486 temp reads, 3867 temp writes, 30 MB external merge
• transactional index-only attempt still sorted all candidates: 1508.137 ms with temp I/O
• SQL rewrite plus transaction-local index: 389.627 ms, 97544 shared hits, 3805 reads, no temp I/O
• the temporary proof index was rolled back and confirmed absent; canonical source rows and volumes were not changed
• both producer modes returned the same ordered candidate rows as feat(orchestrator): scope endpoint routing selector #781 (200/200 for ordinary backfill; 0/0 for case-analysis admission)

Verification

• uv run --extra dev --extra backend pytest -q tests/test_post_content_queue.py tests/test_queue_post_content_backfill_script.py tests/test_post_content_backfill_endpoint.py tests/test_post_content_backfill_schema.py tests/test_migration_replay.py (59 passed)
• git diff --check HEAD^ HEAD

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