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/go.mod

@@ -6,11 +6,8 @@ require (
 	aidanwoods.dev/go-paseto v1.6.0
 	github.com/gin-gonic/gin v1.12.0
 	github.com/golang-migrate/migrate/v4 v4.19.1
-	github.com/google/uuid v1.6.0
 	github.com/joho/godotenv v1.5.1
-	github.com/ledongthuc/pdf v0.0.0-20250511090121-5959a4027728
 	github.com/lib/pq v1.12.3
-	golang.org/x/crypto v0.52.0
 )
 
 require (
@@ -39,6 +36,7 @@ require (
 	github.com/ugorji/go/codec v1.3.1 // indirect
 	go.mongodb.org/mongo-driver/v2 v2.5.0 // indirect
 	golang.org/x/arch v0.22.0 // indirect
+	golang.org/x/crypto v0.52.0 // indirect
 	golang.org/x/net v0.54.0 // indirect
 	golang.org/x/sys v0.45.0 // indirect
 	golang.org/x/text v0.37.0 // indirect