feat(backend): bootstrap Go module, config, and data plane

Introduce configuration validation, Postgres migrations, Redis/storage adapters, and the seed sample video.

backend/internal/db/migrations/0002_chat_attachments.sql

@@ -0,0 +1,5 @@
+ALTER TABLE chat_messages
+    ADD COLUMN IF NOT EXISTS attachment_type TEXT,
+    ADD COLUMN IF NOT EXISTS attachment_url  TEXT,
+    ADD COLUMN IF NOT EXISTS attachment_name TEXT,
+    ADD COLUMN IF NOT EXISTS attachment_size BIGINT;