Upload pipeline: staging, MD5 auto-match, IQDB, three-column board

Backend:
- TempUpload model: staged files (pending / visual_match / completed /
  error) with resolution, e621 payload, custom metadata and IQDB data
- Files land in a temp folder and only move into the watched library
  folder once resolved; duplicates resolve immediately without a copy
- Endpoints: stage (multipart), list, retrieve, temp file, IQDB save,
  resolve (link to post or custom metadata), discard/dismiss
- cleanup_temp_uploads command for old staged files
- Replaces the old direct-to-library upload endpoint

Frontend:
- Upload page is now a three-column board (Pending & Unmatched /
  Visual Similarity Detected / Auto-uploaded & Indexed)
- After upload: MD5s are batch-checked against e621 and matches
  auto-complete with full post metadata; remaining files run through
  IQDB and move to the similarity column when candidates exist
- Metadata modal with IQDB candidates, post-ID linking and custom
  tags/rating/notes; discard and dismiss actions
- e621 client gains fetchPostsByMd5 and iqdbSearch helpers

Roadmap updated with the completed upload items.

backend/apps/library/models.py

@@ -1,6 +1,8 @@
 from django.conf import settings
 from django.db import models
 
+import uuid
+
 
 class MediaItem(models.Model):
     """A logical media file, identified by its MD5 fingerprint."""
@@ -57,3 +59,59 @@ class MediaLocation(models.Model):
 
     def __str__(self):
         return self.rel_path
+
+
+class TempUpload(models.Model):
+    """A staged upload that has not been accepted into the library yet."""
+
+    STATUS_PENDING = "pending"
+    STATUS_VISUAL_MATCH = "visual_match"
+    STATUS_COMPLETED = "completed"
+    STATUS_ERROR = "error"
+    STATUS_CHOICES = [
+        (STATUS_PENDING, "Pending"),
+        (STATUS_VISUAL_MATCH, "Visual Similarity Detected"),
+        (STATUS_COMPLETED, "Completed"),
+        (STATUS_ERROR, "Error"),
+    ]
+
+    RESOLUTION_AUTO_MD5 = "auto_md5"
+    RESOLUTION_DUPLICATE = "duplicate"
+    RESOLUTION_LINKED = "linked"
+    RESOLUTION_CUSTOM = "custom"
+
+    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
+    user = models.ForeignKey(
+        settings.AUTH_USER_MODEL,
+        on_delete=models.CASCADE,
+        related_name="temp_uploads",
+    )
+    file = models.FileField(upload_to="uploads/temp/", blank=True)
+    original_filename = models.CharField(max_length=255)
+    md5 = models.CharField(max_length=32, db_index=True)
+    size = models.BigIntegerField(default=0)
+    status = models.CharField(
+        max_length=20, choices=STATUS_CHOICES, default=STATUS_PENDING
+    )
+    resolution = models.CharField(max_length=20, blank=True, default="")
+    e621_post_id = models.IntegerField(null=True, blank=True)
+    e621_data = models.JSONField(null=True, blank=True)
+    custom_rating = models.CharField(max_length=1, blank=True, default="")
+    custom_tags = models.JSONField(default=list, blank=True)
+    custom_notes = models.TextField(blank=True, default="")
+    iqdb_data = models.JSONField(null=True, blank=True)
+    library_item = models.ForeignKey(
+        MediaItem,
+        null=True,
+        blank=True,
+        on_delete=models.SET_NULL,
+        related_name="temp_uploads",
+    )
+    created_at = models.DateTimeField(auto_now_add=True)
+    updated_at = models.DateTimeField(auto_now=True)
+
+    class Meta:
+        ordering = ["-created_at"]
+
+    def __str__(self):
+        return f"{self.original_filename} ({self.status})"