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/000004_create_operations.up.sql

@@ -0,0 +1,14 @@
+CREATE TABLE operations (
+    id BIGSERIAL PRIMARY KEY,
+    device_id TEXT NOT NULL REFERENCES devices(device_id) ON DELETE CASCADE,
+    operation_id TEXT NOT NULL CHECK (operation_id ~ '^[0-9a-f]{32}$'),
+    op_type TEXT NOT NULL,
+    payload JSONB NOT NULL,
+    status TEXT NOT NULL DEFAULT 'applied' CHECK (status IN ('applied', 'failed')),
+    error_code TEXT,
+    applied_at TIMESTAMPTZ DEFAULT NOW(),
+    created_at TIMESTAMPTZ DEFAULT NOW(),
+    CONSTRAINT operations_unique_per_device UNIQUE (device_id, operation_id)
+);
+
+CREATE INDEX idx_operations_device_created ON operations(device_id, created_at);