feat(admin-brand): add question expansion service and related functionality

- Implemented QuestionExpansionService for generating and materializing questions based on combinations and AI distillation.
- Added question metadata classification logic to infer intent and layer of questions.
- Created API handlers for question expansion operations including combination preview, AI distillation, metadata classification, and materialization.
- Introduced database migrations to support new question-related fields and constraints in brand_questions and brand_keywords tables.
- Added caching mechanism for AI distillation results to improve performance.
- Defined JSON schemas for question distillation responses to ensure data integrity.

server/migrations/20260512143000_add_question_expansion_metadata.down.sql

@@ -0,0 +1,19 @@
+DROP INDEX IF EXISTS uk_brand_question_text_active;
+DROP INDEX IF EXISTS idx_brand_questions_intent;
+
+ALTER TABLE brand_questions
+    DROP CONSTRAINT IF EXISTS ck_brand_questions_layer,
+    DROP CONSTRAINT IF EXISTS ck_brand_questions_intent,
+    DROP CONSTRAINT IF EXISTS ck_brand_questions_source,
+    DROP COLUMN IF EXISTS source,
+    DROP COLUMN IF EXISTS intent,
+    DROP COLUMN IF EXISTS layer;
+
+DROP INDEX IF EXISTS idx_brand_keywords_internal_source;
+
+ALTER TABLE brand_keywords
+    DROP CONSTRAINT IF EXISTS ck_brand_keywords_layer,
+    DROP CONSTRAINT IF EXISTS ck_brand_keywords_source,
+    DROP COLUMN IF EXISTS source,
+    DROP COLUMN IF EXISTS seed_word,
+    DROP COLUMN IF EXISTS layer;

server/migrations/20260512143000_add_question_expansion_metadata.up.sql

@@ -0,0 +1,44 @@
+ALTER TABLE brand_keywords
+    ADD COLUMN IF NOT EXISTS layer VARCHAR(8) NOT NULL DEFAULT 'L2',
+    ADD COLUMN IF NOT EXISTS seed_word VARCHAR(200),
+    ADD COLUMN IF NOT EXISTS source VARCHAR(32) NOT NULL DEFAULT 'manual';
+
+ALTER TABLE brand_keywords
+    DROP CONSTRAINT IF EXISTS ck_brand_keywords_layer,
+    DROP CONSTRAINT IF EXISTS ck_brand_keywords_source;
+
+ALTER TABLE brand_keywords
+    ADD CONSTRAINT ck_brand_keywords_layer
+        CHECK (layer IN ('L1','L2','L3','L4','L5')),
+    ADD CONSTRAINT ck_brand_keywords_source
+        CHECK (source IN ('manual','combination','ai_distill','auto'));
+
+CREATE INDEX IF NOT EXISTS idx_brand_keywords_internal_source
+    ON brand_keywords (tenant_id, brand_id, source)
+    WHERE deleted_at IS NULL;
+
+ALTER TABLE brand_questions
+    ADD COLUMN IF NOT EXISTS layer VARCHAR(8) NOT NULL DEFAULT 'L2',
+    ADD COLUMN IF NOT EXISTS intent VARCHAR(16) NOT NULL DEFAULT 'informational',
+    ADD COLUMN IF NOT EXISTS source VARCHAR(32) NOT NULL DEFAULT 'manual';
+
+ALTER TABLE brand_questions
+    DROP CONSTRAINT IF EXISTS ck_brand_questions_layer,
+    DROP CONSTRAINT IF EXISTS ck_brand_questions_intent,
+    DROP CONSTRAINT IF EXISTS ck_brand_questions_source;
+
+ALTER TABLE brand_questions
+    ADD CONSTRAINT ck_brand_questions_layer
+        CHECK (layer IN ('L1','L2','L3','L4','L5')),
+    ADD CONSTRAINT ck_brand_questions_intent
+        CHECK (intent IN ('informational','evaluative','decisional')),
+    ADD CONSTRAINT ck_brand_questions_source
+        CHECK (source IN ('manual','combination','ai_distill'));
+
+CREATE INDEX IF NOT EXISTS idx_brand_questions_intent
+    ON brand_questions (tenant_id, brand_id, intent)
+    WHERE deleted_at IS NULL;
+
+CREATE UNIQUE INDEX IF NOT EXISTS uk_brand_question_text_active
+    ON brand_questions (tenant_id, brand_id, lower(btrim(question_text)))
+    WHERE deleted_at IS NULL;