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/0010_chat_pins.sql

@@ -0,0 +1,5 @@
+ALTER TABLE chat_messages ADD COLUMN IF NOT EXISTS pinned_at TIMESTAMPTZ;
+
+CREATE INDEX IF NOT EXISTS idx_chat_messages_pinned
+    ON chat_messages (user_id, mentor_id, pinned_at DESC)
+    WHERE pinned_at IS NOT NULL;