feat: auto-narasi LLM (OmniRoute sonnet-5) — hero briefing + narasi_awam report, migrasi 0006

backend/internal/store/migrations/0006_narasi.sql

@@ -0,0 +1,4 @@
+-- FlowSight schema v6: cached LLM narration for briefings.
+-- narasi = 3 kalimat santai Indonesia hasil poles LLM (OmniRoute),
+-- best-effort: kosong berarti FE pakai ringkasan rules.
+ALTER TABLE briefings ADD COLUMN narasi TEXT NOT NULL DEFAULT '';