Phase 3: J-IDs, ownership, roles, guest safety, adaptive detail, download

Backend:
- User.role (user/uploader/staff) with can_upload; uploads and downloads
  gated to uploader+; owners and staff can edit their items
- MediaItem.uploaded_by plus J-<id> identity (serializer, admin,
  scan_files --user, first superuser as default owner)
- API resolves J-<id>, bare numeric ids and MD5s; neighbors and lookup
  return j_ids
- Guest safety: mirror e621's anonymous default blacklist into Redis
  (parses comments, negations and wildcards), flag hidden_from_guests
  and filter lists, details and lookups for anonymous users
- POST /api/online/downloads/ writes an e621 file into the watched
  folder and indexes it for the uploader
- MariaDB + Redis via docker compose (host ports 3307/6380), PyMySQL
  driver shim, Redis cache replacing the file cache; SQLite data
  dumped and loaded into MariaDB

Frontend:
- Single /detail/:itemId route with an adaptive shell: J-<id> renders
  the library item, bare numbers render the e621 post
- Legacy /view/<md5> and /online/view/<id> redirect to canonical URLs
- Cards expose J-IDs; library custom-data editor is read-only for
  non-owners
- Role gating: Upload hidden/blocked for regular users, account shows
  the role, guest hint on the library

backend/apps/library/models.py

@@ -1,3 +1,4 @@
+from django.conf import settings
 from django.db import models
 
 
@@ -10,6 +11,15 @@ class MediaItem(models.Model):
     rating = models.CharField(max_length=1, blank=True, default="")
     tags = models.JSONField(default=list, blank=True)
     notes = models.TextField(blank=True, default="")
+    uploaded_by = models.ForeignKey(
+        settings.AUTH_USER_MODEL,
+        null=True,
+        blank=True,
+        on_delete=models.SET_NULL,
+        related_name="uploads",
+    )
+    # Guests never see items whose tags hit the mirrored default blacklist.
+    hidden_from_guests = models.BooleanField(default=False, db_index=True)
     created_at = models.DateTimeField(auto_now_add=True)
     updated_at = models.DateTimeField(auto_now=True)
 
@@ -17,7 +27,13 @@ class MediaItem(models.Model):
         ordering = ["-created_at"]
 
     def __str__(self):
-        return self.md5
+        return f"J-{self.pk} ({self.md5})"
+
+    def save(self, *args, **kwargs):
+        from .guest_filter import item_hidden_from_guests
+
+        self.hidden_from_guests = item_hidden_from_guests(self.tags)
+        super().save(*args, **kwargs)
 
 
 class MediaLocation(models.Model):