fix(supabase): make GUC-set migration hosted-safe; neutralize obsolete N+1 migration

[BigSimmo]

Summary

Applying the pending audit-follow-up migrations against the hosted Clinical KB Supabase project surfaced two migration-set hazards that would break a fresh migration replay / preview branch (they don't affect the already-live DB, where the safe halves were hand-applied). This PR makes the migration set hosted-safe with zero runtime-behaviour change.

1. 20260702160000_fix_invoke_agent_url_to_guc — guard the ALTER DATABASE SET

Hosted Supabase denies alter database ... set app.indexing_v3_agent_base_url to the migration role (42501 insufficient_privilege), which aborts the whole migration on replay. Wrapped it in a DO block that swallows insufficient_privilege (still sets the GUC on self-hosted / local where the role is permitted). The function body is unchanged and keeps its current_setting('app.indexing_v3_agent_base_url', true) → hardcoded production-URL fallback, so invoke_indexing_v3_agent behaves identically whether or not the GUC is set. The mirrored statement in supabase/schema.sql gets the same guard.

2. 20260702170000_fix_match_chunks_text_n1 — neutralize (documented no-op)

Obsolete and hazardous:

• It would error: it adds a lexical_score output column to match_document_chunks_text via CREATE OR REPLACE, which is illegal on an existing RETURNS TABLE function without a DROP first.
• It would regress live: the deployed match_document_chunks_text already batches the N+1 label/summary lookups and carries a superior title-boost dual-path impl (chunk_seed / title_docs / title_matches) this file lacks.

Replaced the body with a documented select 1; no-op, kept for migration ordering. Full retrieval-RPC drift reconciliation (the known hybrid-RPC drift) into schema.sql + migrations remains a golden-eval-gated backlog item — deliberately out of scope here.

3. Test

tests/supabase-schema.test.ts now asserts the guarded GUC-set form (alter database %I set ... = %L + when insufficient_privilege then) instead of the removed bare literal.

Verification

• tests/supabase-schema.test.ts: 29/29 (directly validates the guarded ALTER DATABASE form).
• verify:cheap (lint + typecheck + unit): green — 905/907, with 2 unrelated 15s flaky timeouts (api-validation-contract, rag-answer-fallback) that pass 45/45 in isolation; a SQL/schema-string change cannot affect API-route or RAG-fallback logic.
• prettier --check clean on the changed .ts.

Scope / risk

SQL-only + one test assertion. No application code, no live-DB change, no edge-function change. Does not re-activate enrichment or touch the retrieval-RPC drift.

Clinical governance preflight

• Patient-data / PHI touched? No.
• Retrieval / ranking / answer-generation behaviour changed? No — match_document_chunks_text is left exactly as deployed; the invoke-agent function keeps identical fallback behaviour.
• Golden retrieval eval impact? None expected (no retrieval path changed); drift reconciliation that would need eval:retrieval:quality 23/23 is explicitly deferred to backlog.
• Deployment classification: repo hygiene / migration-replay safety only.

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