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/20260331100023_create_brand_question_versions.up.sql

@@ -1,11 +1 @@
-CREATE TABLE 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 idx_question_hash ON brand_question_versions(question_hash);
+-- No-op: question versioning was removed during development.