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/20260408110000_flatten_brand_questions.down.sql

@@ -0,0 +1,37 @@
+ALTER TABLE brand_questions
+    ADD COLUMN IF NOT EXISTS current_version_id BIGINT;
+
+CREATE TABLE IF NOT EXISTS brand_question_versions (
+    id            BIGSERIAL PRIMARY KEY,
+    question_id   BIGINT NOT NULL REFERENCES brand_questions(id),
+    question_text TEXT NOT NULL,
+    question_hash VARCHAR(64) NOT NULL,
+    version_no    INT NOT NULL,
+    is_active     BOOLEAN NOT NULL DEFAULT true,
+    created_at    TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    CONSTRAINT uk_question_version_no UNIQUE (question_id, version_no)
+);
+
+CREATE INDEX IF NOT EXISTS idx_question_hash ON brand_question_versions(question_hash);
+
+INSERT INTO brand_question_versions (question_id, question_text, question_hash, version_no, is_active)
+SELECT q.id,
+       q.question_text,
+       md5(q.question_text),
+       1,
+       true
+FROM brand_questions q
+WHERE NOT EXISTS (
+    SELECT 1
+    FROM brand_question_versions v
+    WHERE v.question_id = q.id
+);
+
+UPDATE brand_questions q
+SET current_version_id = v.id
+FROM brand_question_versions v
+WHERE v.question_id = q.id
+  AND v.version_no = 1;
+
+ALTER TABLE brand_questions
+    DROP COLUMN IF EXISTS question_text;