feat(backend): migrations Postgres testées (users, devices, resources, operations, ocr_jobs) + migrate au boot

- golang-migrate embarqué via embed (db/migrations/*.sql), MigrateDatabase(url) au boot
- ids TEXT 32-hex avec CHECK ~ '^[0-9a-f]{32}$' ; operations idempotentes UNIQUE(device_id, operation_id)
- harness db/migrations_test.go (up → assertions schéma → down, skip si PG indisponible)
- DB dev fraîche vaultdrop_dev (la DB vaultdrop héberge un prototype V2 abandonné)

backend/db/migrations/000002_create_devices.up.sql

@@ -0,0 +1,9 @@
+CREATE TABLE devices (
+    device_id TEXT PRIMARY KEY CHECK (device_id ~ '^[0-9a-f]{32}$'),
+    user_id TEXT REFERENCES users(id) ON DELETE SET NULL,
+    created_at TIMESTAMPTZ DEFAULT NOW(),
+    registered_at TIMESTAMPTZ DEFAULT NOW(),
+    last_seen_at TIMESTAMPTZ
+);
+
+CREATE INDEX idx_devices_user ON devices(user_id) WHERE user_id IS NOT NULL;