feat(import): track release date evidence tiers

db/migrations/006_model_release_date_metadata.sql

@@ -0,0 +1,34 @@
+-- Phase 2.1: 模型发布日期证据元数据
+-- 区分一级官方发布日期与二级权威佐证日期，避免混淆 source_url 与发布日期证据层级
+
+DO $$
+BEGIN
+    IF NOT EXISTS (SELECT 1 FROM information_schema.columns WHERE table_name='models' AND column_name='date_confidence') THEN
+        ALTER TABLE models ADD COLUMN date_confidence TEXT NOT NULL DEFAULT 'unknown';
+    END IF;
+
+    IF NOT EXISTS (SELECT 1 FROM information_schema.columns WHERE table_name='models' AND column_name='date_source_kind') THEN
+        ALTER TABLE models ADD COLUMN date_source_kind TEXT NOT NULL DEFAULT 'unknown';
+    END IF;
+END $$;
+
+DO $$
+BEGIN
+    IF NOT EXISTS (SELECT 1 FROM pg_constraint WHERE conname='chk_models_date_confidence') THEN
+        ALTER TABLE models
+            ADD CONSTRAINT chk_models_date_confidence
+            CHECK (date_confidence IN ('official_primary', 'secondary_authoritative', 'inferred', 'unknown'));
+    END IF;
+
+    IF NOT EXISTS (SELECT 1 FROM pg_constraint WHERE conname='chk_models_date_source_kind') THEN
+        ALTER TABLE models
+            ADD CONSTRAINT chk_models_date_source_kind
+            CHECK (date_source_kind IN ('official_announcement', 'official_product_page', 'secondary_authoritative_report', 'catalog_backfill', 'unknown'));
+    END IF;
+END $$;
+
+CREATE INDEX IF NOT EXISTS idx_models_date_confidence ON models(date_confidence);
+CREATE INDEX IF NOT EXISTS idx_models_date_source_kind ON models(date_source_kind);
+
+COMMENT ON COLUMN models.date_confidence IS '发布日期证据置信度：official_primary / secondary_authoritative / inferred / unknown';
+COMMENT ON COLUMN models.date_source_kind IS '发布日期证据来源类型：official_announcement / official_product_page / secondary_authoritative_report / catalog_backfill / unknown';