fix(images): percent-encode original-image URLs ('#' in paths 404'd)

An image whose on-disk path contains '#' (post folders like 'BLUE#59')
served its hash-named thumbnail fine but 404'd the original: the unencoded
'#' in image_url was parsed by the browser as a URL fragment, so
'#59/01_timelapse.jpg' never reached the /images route. Add a shared
image_url(path) helper that percent-encodes the path (safe='/') and route
the 3 raw builders (gallery detail + 2 in series) through it. Not a
cleanup-tool deletion — the file is on disk; only the URL was wrong.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/services/series_service.py

@@ -19,7 +19,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 from ..models import Artist, ImageRecord, Post, Tag, TagKind
 from ..models.series_chapter import SeriesChapter
 from ..models.series_page import SeriesPage
-from .gallery_service import thumbnail_url
+from .gallery_service import image_url, thumbnail_url
 from .page_number_parser import parse_page_range
 
 
@@ -194,7 +194,7 @@ class SeriesService:
                 "thumbnail_url": thumbnail_url(
                     r.thumbnail_path, r.sha256, r.mime
                 ),
-                "image_url": f"/images/{r.path.split('/images/', 1)[-1]}",
+                "image_url": image_url(r.path),
                 "source_post": (
                     {"id": r.primary_post_id, "title": r.post_title}
                     if r.primary_post_id is not None
@@ -282,7 +282,7 @@ class SeriesService:
                     "thumbnail_url": thumbnail_url(
                         r.thumbnail_path, r.sha256, r.mime
                     ),
-                    "image_url": f"/images/{r.path.split('/images/', 1)[-1]}",
+                    "image_url": image_url(r.path),
                 }
             )
             # Group start page = the post's parsed starting page (stored on the