feat: add AI chat for repports

backend/internal/database/migrations/000010_report_messages.up.sql

@@ -0,0 +1,8 @@
+CREATE TABLE report_messages (
+    id         UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    report_id  UUID NOT NULL REFERENCES reports(id) ON DELETE CASCADE,
+    role       VARCHAR(10) NOT NULL CHECK (role IN ('user', 'assistant')),
+    content    TEXT NOT NULL,
+    status     VARCHAR(10) NOT NULL DEFAULT 'done' CHECK (status IN ('done', 'generating', 'error')),
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);