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/0006_username.sql

@@ -0,0 +1,19 @@
+ALTER TABLE users ADD COLUMN IF NOT EXISTS username TEXT;
+
+UPDATE users
+SET username = lower(split_part(email, '@', 1))
+WHERE username IS NULL OR username = '';
+
+UPDATE users u
+SET username = u.username || '-' || u.id::text
+WHERE u.id IN (
+    SELECT id FROM (
+        SELECT id,
+               ROW_NUMBER() OVER (PARTITION BY username ORDER BY id) AS rn
+        FROM users
+    ) ranked
+    WHERE ranked.rn > 1
+);
+
+ALTER TABLE users ALTER COLUMN username SET NOT NULL;
+CREATE UNIQUE INDEX IF NOT EXISTS users_username_uidx ON users (lower(username));