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/0008_prod_indexes.sql

@@ -0,0 +1,8 @@
+-- Hot-path indexes and heartbeat retention helpers.
+CREATE INDEX IF NOT EXISTS idx_quiz_attempts_user_lesson ON quiz_attempts (user_id, lesson_id, created_at DESC);
+CREATE INDEX IF NOT EXISTS idx_chat_messages_pair ON chat_messages (user_id, mentor_id, id DESC);
+CREATE INDEX IF NOT EXISTS idx_lesson_progress_user ON lesson_progress (user_id);
+CREATE INDEX IF NOT EXISTS idx_watch_heartbeats_created ON watch_heartbeats (created_at);
+CREATE INDEX IF NOT EXISTS idx_events_starts ON events (starts_at) WHERE is_active;
+CREATE INDEX IF NOT EXISTS idx_refresh_tokens_user ON refresh_tokens (user_id) WHERE revoked_at IS NULL;
+CREATE INDEX IF NOT EXISTS idx_xp_events_user ON xp_events (user_id, created_at DESC);