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/0004_chat_reactions.sql

@@ -0,0 +1,9 @@
+CREATE TABLE IF NOT EXISTS chat_reactions (
+  id         BIGSERIAL PRIMARY KEY,
+  message_id BIGINT  NOT NULL REFERENCES chat_messages(id) ON DELETE CASCADE,
+  user_id    BIGINT  NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+  emoji      TEXT    NOT NULL,
+  created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+  UNIQUE(message_id, user_id, emoji)
+);
+CREATE INDEX IF NOT EXISTS idx_chat_reactions_msg ON chat_reactions(message_id);