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/0011_physical_shipping.sql

@@ -0,0 +1,24 @@
+-- Shipping details + tracking for physical certificates
+
+ALTER TABLE certificate_physical_orders
+    ADD COLUMN IF NOT EXISTS recipient_name TEXT NOT NULL DEFAULT '',
+    ADD COLUMN IF NOT EXISTS phone TEXT NOT NULL DEFAULT '',
+    ADD COLUMN IF NOT EXISTS address TEXT NOT NULL DEFAULT '',
+    ADD COLUMN IF NOT EXISTS city TEXT NOT NULL DEFAULT '',
+    ADD COLUMN IF NOT EXISTS postal_code TEXT NOT NULL DEFAULT '',
+    ADD COLUMN IF NOT EXISTS tracking_code TEXT NOT NULL DEFAULT '';
+
+CREATE TABLE IF NOT EXISTS unlock_requests (
+    id          BIGSERIAL PRIMARY KEY,
+    user_id     BIGINT NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    note        TEXT NOT NULL DEFAULT '',
+    created_at  TIMESTAMPTZ NOT NULL DEFAULT now(),
+    resolved_at TIMESTAMPTZ
+);
+
+CREATE INDEX IF NOT EXISTS unlock_requests_user_created_idx
+    ON unlock_requests (user_id, created_at DESC);
+
+CREATE INDEX IF NOT EXISTS unlock_requests_open_idx
+    ON unlock_requests (created_at DESC)
+    WHERE resolved_at IS NULL;