test: repair integration harness — wire SettingsRegistry, reuse preserved dev admin

Two pre-existing harness breaks found while running the device tests:

- RegisterRoutes dereferences cfg.Settings (auth rate limit) but
  setupTestServer never set it — every integration test nil-panicked at
  route registration. Wire database.NewSettingsRegistry(queries) the
  same way main.go does.
- The cleanup deliberately PRESERVES testuser@tests.bookhoard.internal
  (shared dev admin), but setup then blindly re-INSERTed that user, so
  every run after the first failed on users_email_key. Reuse the user
  when it exists; default collections are created only for a NEW user
  (the preserved admin already has its set).

Also documented the offline runner used on slow links: build the test
binary on the host (CGO_ENABLED=0 go test -c) and execute it inside the
bookhoard-tests image against the compose network — no in-container
module downloads, no image rebuild.

cmd/server/tests/test_helpers_test.go

@@ -544,6 +544,10 @@ func setupTestServer(t *testing.T) *TestServerSetup {
 		AnnotationService:     annotationService,
 		DeviceAuthMiddleware:  deviceAuthMiddleware,
 		LoginTracker:          loginAttemptTracker,
+		// RegisterRoutes dereferences Settings for the auth rate limit;
+		// the main binary wires database.NewSettingsRegistry(queries) —
+		// the test harness must too or route registration nil-panics.
+		Settings: database.NewSettingsRegistry(queries),
 	}
 
 	router.RegisterRoutes(routerConfig)
@@ -588,9 +592,17 @@ func setupTestServer(t *testing.T) *TestServerSetup {
 		}
 	}
 
-	// Create fresh admin test user
+	// The admin test user doubles as the shared dev admin and is
+	// deliberately PRESERVED by the cleanup above — reuse it when it
+	// already exists instead of re-creating (a blind INSERT hits
+	// users_email_key on every run after the first).
 	passwordHash := "$2a$10$JjAtK7PPa1WexQC3AUGe8OXLeuseZ/haN1Mz7emMo6CfOvMiTVXWq"
-	adminUser, err := queries.CreateUser(ctx, database.CreateUserParams{
+	var adminUUID uuid.UUID
+	if existing, gerr := queries.GetUserByEmail(ctx, "testuser@tests.bookhoard.internal"); gerr == nil {
+		adminUUID, err = uuid.FromBytes(existing.ID.Bytes[:])
+		require.NoError(t, err, "Failed to parse admin user UUID")
+	} else {
+		adminUser, cerr := queries.CreateUser(ctx, database.CreateUserParams{
 			Email:        "testuser@tests.bookhoard.internal",
 			Username:     "testuser",
 			PasswordHash: passwordHash,
@@ -598,11 +610,13 @@ func setupTestServer(t *testing.T) *TestServerSetup {
 			LastName:     pgtype.Text{String: "User", Valid: true},
 			Role:         "admin",
 		})
-	require.NoError(t, err, "Failed to create admin test user")
+		require.NoError(t, cerr, "Failed to create admin test user")
-
+		adminUUID, err = uuid.FromBytes(adminUser.ID.Bytes[:])
-	adminUUID, err := uuid.FromBytes(adminUser.ID.Bytes[:])
 		require.NoError(t, err, "Failed to parse admin user UUID")
+		// Default collections are created exactly once per user; the
+		// preserved dev admin already has its set from the first run.
 		createDefaultCollectionsForUser(t, queries, pgtype.UUID{Bytes: adminUUID, Valid: true})
+	}
 
 	// Create fresh regular test user
 	regularUser, err := queries.CreateUser(ctx, database.CreateUserParams{