feat(db): idempotent schema + 'codex migrate' command (audit M-1, T-2)

M-1: schema.sql could not be re-applied (leading CREATE TABLE/INDEX lacked
IF NOT EXISTS), apply_schema was never called, and the live migration just
failed on a missing chunks.section column. Fixes:

- schema.sql: all CREATE TABLE/INDEX now use IF NOT EXISTS — the whole file is
  re-applyable as a no-op.
- codex migrate: new CLI command that applies schema.sql. Connects via
  MIGRATION_DATABASE_URL (falls back to DATABASE_URL) and catches
  InsufficientPrivilege with guidance — because the app role is DML-only and
  core tables are owned by 'postgres' (the privilege dimension found during the
  live migration incident).
- config: migration_database_url (optional privileged connection).
- db: apply_schema docstring corrected (idempotency now true) + privilege note.
- T-2: static test that schema.sql is fully idempotent; migrate privilege-error
  test.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

infra/schema.sql

@@ -13,7 +13,7 @@ CREATE EXTENSION IF NOT EXISTS vector;
 -- ---------------------------------------------------------------------
 -- Layer 1: Papers + full-text chunks
 -- ---------------------------------------------------------------------
-CREATE TABLE papers (
+CREATE TABLE IF NOT EXISTS papers (
     id            TEXT PRIMARY KEY,        -- canonical ID: arXiv ID or DOI
     openalex_id   TEXT UNIQUE,             -- e.g. W2741809807
     bibkey        TEXT UNIQUE,             -- BibTeX key -> .bib + Doxygen @cite
@@ -27,10 +27,10 @@ CREATE TABLE papers (
 );
 
 -- HNSW index for fast approximate nearest-neighbour search on abstracts.
-CREATE INDEX papers_abstract_emb_idx
+CREATE INDEX IF NOT EXISTS papers_abstract_emb_idx
     ON papers USING hnsw (abstract_emb vector_cosine_ops);
 
-CREATE TABLE chunks (
+CREATE TABLE IF NOT EXISTS chunks (
     id        BIGSERIAL PRIMARY KEY,
     paper_id  TEXT REFERENCES papers(id) ON DELETE CASCADE,
     ord       INT NOT NULL,               -- position within the paper
@@ -38,13 +38,13 @@ CREATE TABLE chunks (
     embedding vector(1024)
 );
 
-CREATE INDEX chunks_emb_idx
+CREATE INDEX IF NOT EXISTS chunks_emb_idx
     ON chunks USING hnsw (embedding vector_cosine_ops);
-CREATE INDEX chunks_paper_idx ON chunks (paper_id);
+CREATE INDEX IF NOT EXISTS chunks_paper_idx ON chunks (paper_id);
 
 -- Sparse / keyword hits for exact mathematical terminology (hybrid search).
 -- Dense (above) + full-text (below) combined = robust against math terms.
-CREATE INDEX chunks_fts_idx
+CREATE INDEX IF NOT EXISTS chunks_fts_idx
     ON chunks USING gin (to_tsvector('english', content));
 
 -- ---------------------------------------------------------------------
@@ -53,19 +53,19 @@ CREATE INDEX chunks_fts_idx
 --   edges to not-yet-ingested papers are intentionally preserved —
 --   those are your discovery leads.
 -- ---------------------------------------------------------------------
-CREATE TABLE citations (
+CREATE TABLE IF NOT EXISTS citations (
     citing_id TEXT REFERENCES papers(id) ON DELETE CASCADE,
     cited_id  TEXT NOT NULL,               -- arXiv/DOI/OpenAlex ID of the target
     context   TEXT,                        -- optional: citation context (S2)
     PRIMARY KEY (citing_id, cited_id)
 );
 
-CREATE INDEX citations_cited_idx ON citations (cited_id);
+CREATE INDEX IF NOT EXISTS citations_cited_idx ON citations (cited_id);
 
 -- ---------------------------------------------------------------------
 -- Layer 3: Provenance  (the "cleanly couple" goal)
 -- ---------------------------------------------------------------------
-CREATE TABLE code_links (
+CREATE TABLE IF NOT EXISTS code_links (
     id        BIGSERIAL PRIMARY KEY,
     symbol    TEXT NOT NULL,               -- e.g. 'dec::hodge_star' or 'file.cpp:120'
     paper_id  TEXT REFERENCES papers(id) ON DELETE SET NULL,
@@ -74,8 +74,8 @@ CREATE TABLE code_links (
     added_at  TIMESTAMPTZ DEFAULT now()
 );
 
-CREATE INDEX code_links_symbol_idx ON code_links (symbol);
-CREATE INDEX code_links_paper_idx  ON code_links (paper_id);
+CREATE INDEX IF NOT EXISTS code_links_symbol_idx ON code_links (symbol);
+CREATE INDEX IF NOT EXISTS code_links_paper_idx  ON code_links (paper_id);
 
 -- ---------------------------------------------------------------------
 -- Example query: discovery leads