feat(graphstore): M1 add tier + topic columns to brain_entities (infra#72)

Schema-only change. DDL adds tier + topic on fresh tables and uses
ADD COLUMN IF NOT EXISTS on existing tables (idempotent across pod
restarts). New conditional indexes match the wing/hall pattern.

No behavior change in this commit — UpsertEntity still writes only
the original columns; tier + topic stay '' on every row. M2 plumbs
the parser through. The empty default means existing queries are
untouched until the rest of the chain lands.

Part of infra#72 — brain DIKW tier redesign.

ingestion/internal/graphstore/pg.go

@@ -69,12 +69,24 @@ CREATE TABLE IF NOT EXISTS brain_entities (
     hall       TEXT NOT NULL DEFAULT '',
     doc_path   TEXT NOT NULL,
     title      TEXT NOT NULL DEFAULT '',
+    tier       TEXT NOT NULL DEFAULT '',
+    topic      TEXT NOT NULL DEFAULT '',
     updated_at TIMESTAMPTZ NOT NULL DEFAULT now()
 );
+-- Idempotent migration for clusters created before the DIKW tier
+-- redesign (infra#72). ADD COLUMN IF NOT EXISTS is safe across
+-- repeated startups.
+ALTER TABLE brain_entities
+    ADD COLUMN IF NOT EXISTS tier  TEXT NOT NULL DEFAULT '',
+    ADD COLUMN IF NOT EXISTS topic TEXT NOT NULL DEFAULT '';
 CREATE INDEX IF NOT EXISTS brain_entities_wing_idx
     ON brain_entities (wing) WHERE wing <> '';
 CREATE INDEX IF NOT EXISTS brain_entities_type_idx
     ON brain_entities (type);
+CREATE INDEX IF NOT EXISTS brain_entities_tier_idx
+    ON brain_entities (tier) WHERE tier <> '';
+CREATE INDEX IF NOT EXISTS brain_entities_topic_idx
+    ON brain_entities (topic) WHERE topic <> '';
 
 CREATE TABLE IF NOT EXISTS brain_edges (
     id         BIGSERIAL PRIMARY KEY,