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.

internal/router/router.go

@@ -69,7 +69,7 @@ type Config struct {
 	LoginTracker            *ratelimit.LoginAttemptTracker
 	ScannerHandler          *handlers.Handler
 	JobsHandler             *handlers.JobsHandler
-	SidecarHandler           *handlers.SidecarHandler
+	SidecarHandler          *handlers.SidecarHandler
 	ReaderHandler           *handlers.ReaderHandler
 	LibraryService          *services.LibraryService
 }
@@ -113,6 +113,31 @@ func createJWTMiddleware(cfg *Config) echo.MiddlewareFunc {
 				return echo.NewHTTPError(http.StatusBadRequest, "invalid user ID in token")
 			}
 
+			// Ghost-session guard: a signature-valid token for a user that
+			// no longer exists must not authenticate (deleted users kept
+			// full access until token expiry before this check). A
+			// primary-key lookup per request — instant revocation, no TTL
+			// semantics; the guard also covers the SSR groups that used to
+			// chain ensureUserExistsMiddleware.
+			if !userExists(cfg, userUUID) {
+				log.Printf("Ghost session rejected: user %s no longer exists", userIDStr)
+				c.SetCookie(&http.Cookie{
+					Name:     "token",
+					Value:    "",
+					Expires:  time.Now().Add(-24 * time.Hour),
+					Path:     "/",
+					HttpOnly: true,
+					SameSite: http.SameSiteLaxMode,
+				})
+				if wantsHTML(c.Request().Header) {
+					return c.Redirect(http.StatusFound, "/login?session=invalid")
+				}
+				return c.JSON(http.StatusUnauthorized, map[string]string{
+					"error":   "user_not_found",
+					"message": "This account no longer exists. Please log in again.",
+				})
+			}
+
 			c.Set("user", database.Users{
 				ID:       pgtype.UUID{Bytes: userUUID, Valid: true},
 				Email:    claims["user_email"].(string),
@@ -138,43 +163,12 @@ func createJWTMiddleware(cfg *Config) echo.MiddlewareFunc {
 	})
 }
 
-// ensureUserExistsMiddleware checks if the authenticated user still exists in the database
-func ensureUserExistsMiddleware(cfg *Config) echo.MiddlewareFunc {
-	return func(next echo.HandlerFunc) echo.HandlerFunc {
-		return func(c *echo.Context) error {
-			userIDStr, ok := c.Get("user_id").(string)
-			if !ok {
-				return next(c)
-			}
-
-			userUUID, err := uuid.Parse(userIDStr)
-			if err != nil {
-				log.Printf("Invalid UUID in user existence check: %v", err)
-				return next(c)
-			}
-
-			// Check if user exists in database
-			_, err = cfg.Queries.GetUser(c.Request().Context(), uuidToPGType(userUUID))
-			if err != nil {
-				log.Printf("User not found in database: %s", userIDStr)
-
-				// Clear invalid cookie
-				c.SetCookie(&http.Cookie{
-					Name:     "token",
-					Value:    "",
-					Expires:  time.Now().Add(-24 * time.Hour),
-					Path:     "/",
-					HttpOnly: true,
-					SameSite: http.SameSiteLaxMode,
-				})
-
-				// Redirect to login with session=invalid
-				return c.Redirect(http.StatusFound, "/login?session=invalid")
-			}
-
-			return next(c)
-		}
-	}
+// userExists checks the users table for the token's claimed user. Runs on
+// every authenticated request: a primary-key lookup, cheap at this scale,
+// and it makes deletion revocable the moment it happens.
+func userExists(cfg *Config, id uuid.UUID) bool {
+	_, err := cfg.Queries.GetUser(context.Background(), uuidToPGType(id))
+	return err == nil
 }
 
 // wantsHTML determines if the request expects HTML response