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/0021_challenges.sql

@@ -0,0 +1,25 @@
+CREATE TABLE IF NOT EXISTS challenges (
+    id          BIGSERIAL PRIMARY KEY,
+    title       TEXT NOT NULL,
+    description TEXT NOT NULL DEFAULT '',
+    target_xp   INT NOT NULL CHECK (target_xp > 0),
+    starts_at   TIMESTAMPTZ NOT NULL,
+    ends_at     TIMESTAMPTZ NOT NULL,
+    is_active   BOOLEAN NOT NULL DEFAULT true,
+    created_by  BIGINT REFERENCES users(id) ON DELETE SET NULL,
+    created_at  TIMESTAMPTZ NOT NULL DEFAULT now(),
+    CHECK (ends_at >= starts_at)
+);
+
+CREATE INDEX IF NOT EXISTS idx_challenges_active_window
+    ON challenges (is_active, starts_at, ends_at);
+
+CREATE TABLE IF NOT EXISTS challenge_progress (
+    challenge_id BIGINT NOT NULL REFERENCES challenges(id) ON DELETE CASCADE,
+    user_id      BIGINT NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    xp           INT NOT NULL DEFAULT 0 CHECK (xp >= 0),
+    updated_at   TIMESTAMPTZ NOT NULL DEFAULT now(),
+    PRIMARY KEY (challenge_id, user_id)
+);
+
+CREATE INDEX IF NOT EXISTS idx_challenge_progress_user ON challenge_progress (user_id);