feat: Introduce AI Brand Monitoring System V5 technical design document

- Added comprehensive technical design document for AI Brand Monitoring System V5, outlining system architecture, data models, sampling strategies, and monitoring protocols.
- Key changes include a shift to a sampling-based trend monitoring approach, updated data collection and storage strategies, and new metrics for performance evaluation.
- Implemented migration scripts to support the flattening of brand questions and versioning of question texts, ensuring historical data integrity and version control.

server/migrations/20260331100022_create_brand_questions.up.sql

@@ -1,12 +1,12 @@
 CREATE TABLE brand_questions (
-    id                 BIGSERIAL PRIMARY KEY,
-    tenant_id          BIGINT NOT NULL REFERENCES tenants(id),
-    brand_id           BIGINT NOT NULL REFERENCES brands(id),
-    keyword_id         BIGINT NOT NULL REFERENCES brand_keywords(id),
-    current_version_id BIGINT,
-    status             VARCHAR(20) NOT NULL DEFAULT 'active',
-    created_at         TIMESTAMPTZ NOT NULL DEFAULT NOW(),
-    updated_at         TIMESTAMPTZ NOT NULL DEFAULT NOW(),
-    deleted_at         TIMESTAMPTZ
+    id            BIGSERIAL PRIMARY KEY,
+    tenant_id     BIGINT NOT NULL REFERENCES tenants(id),
+    brand_id      BIGINT NOT NULL REFERENCES brands(id),
+    keyword_id    BIGINT NOT NULL REFERENCES brand_keywords(id),
+    question_text TEXT NOT NULL,
+    status        VARCHAR(20) NOT NULL DEFAULT 'active',
+    created_at    TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at    TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    deleted_at    TIMESTAMPTZ
 );
 CREATE INDEX idx_brand_questions_keyword ON brand_questions(keyword_id);