feat(server): ghost-session guard — JWTs for deleted users stop authenticating

The JWT middleware validated signature + expiry and fabricated the
request user purely from token claims, so a signature-valid token for a
DELETED user kept full API access until token expiry — observed live on
hardware (a phone session survived the test suite's user-table
recreation and kept serving its true empty sets; open handoff bug 1).

An ensureUserExistsMiddleware already existed but was only wired on the
frontend SSR group (frontend.go) — every /api group used bare
createJWTMiddleware. The existence check now lives INSIDE
createJWTMiddleware's SuccessHandler so every protected group (SSR and
API alike) gets it:

- After claim validation, GetUser by the token's user_id; a missing row
  clears the token cookie and answers 401 user_not_found for API calls
  (the app's TokenAuthenticator then re-logins with stored credentials)
  or redirects to /login?session=invalid for page requests — same
  shape-aware split the error handler already uses.
- Cost: one primary-key lookup per authenticated request (~0.2ms local);
  instant revocation, no TTL semantics. The redundant SSR-only
  middleware is deleted (its redirect behavior is preserved inside the
  guard).

The guard exposed two processing-issues tests that were unknowingly
RELYING on ghost semantics: they nested a second setupTestServer whose
bootstrap deleted the first setup's regular user, then asserted 403
from the dead token. De-nested: a createLibraryDirect helper inserts
the library under the live setup's user, no second bootstrap.

New TestDeletedUserTokenRejected pins the contract: token works while
the user exists -> 401 user_not_found after deletion. Full suite: 219
groups, zero failures.

cmd/server/tests/test_helpers_test.go

@@ -794,6 +794,26 @@ func getTestUserID(t *testing.T, db *database.Queries) uuid.UUID {
 	return userUUID
 }
 
+// createLibraryDirect inserts a library owned by the given user WITHOUT a
+// nested test-server setup: a second setupTestServer bootstraps away the
+// first setup's users, and the ghost-session guard rightly rejects the
+// outer test's tokens from then on.
+func createLibraryDirect(t *testing.T, db *database.Queries, ownerID uuid.UUID, name, libraryType string) string {
+	ctx := context.Background()
+	libraryTypeRow, err := db.GetLibraryTypeByName(ctx, libraryType)
+	require.NoError(t, err, "Should find library type")
+	library, err := db.CreateLibrary(ctx, database.CreateLibraryParams{
+		Name:             name,
+		Description:      pgtype.Text{String: "Test library description", Valid: true},
+		LibraryTypeID:    libraryTypeRow.ID,
+		CreatedByAdminID: pgtype.UUID{Bytes: [16]byte(ownerID), Valid: true},
+	})
+	require.NoError(t, err, "Should create library")
+	libraryUUID, err := uuid.FromBytes(library.ID.Bytes[0:16])
+	require.NoError(t, err, "Should parse library ID")
+	return libraryUUID.String()
+}
+
 func getRegularUserID(t *testing.T, db *database.Queries) uuid.UUID {
 	ctx := context.Background()
 	user, err := db.GetUserByEmail(ctx, "testregularuser@tests.bookhoard.internal")