feat(16-02): add migration 099 — indicators.metadata JSONB (D-07)

- Nullable JSONB column, no backfill needed (no rows exist yet)
- Applied to the running dev DB (docker exec against pulse-postgres);
  Postgres init only runs migrations on first volume boot

migrations/099_indicators_metadata.sql

@@ -0,0 +1,16 @@
+-- =============================================================================
+-- indicators.metadata JSONB (Phase 16 — D-07)
+-- =============================================================================
+-- Lets an attachment-hash indicator carry filename/content-type/size, or a
+-- URL indicator carry which message part it came from, without duplicating
+-- that context into the parent messages row.
+--
+-- Nullable, no default — existing rows (there are none yet for this table)
+-- are unaffected; this is a pure additive change.
+-- =============================================================================
+
+ALTER TABLE indicators
+  ADD COLUMN IF NOT EXISTS metadata JSONB;
+
+COMMENT ON COLUMN indicators.metadata IS
+  'Per-indicator context (e.g. attachment filename/content-type/size for attachment-hash indicators, or source message part for URL indicators). Nullable — no backfill required.';