feat: serve width-bounded thumbnail variants for image grids

Add /files/{key}?w=<px>: the endpoint decodes JPEG/PNG sources, downscales to
a width snapped to a small allow-list (aspect-preserving, never upscaling),
caches the result under a thumbs/ prefix, and serves it — so cover grids
download ~KB instead of the 1–5 MB original. Cover handlers purge cached
variants when a cover (whose key is reused) is replaced or deleted. Frontend
grids use new thumbUrl/thumbSrcset helpers (MangaCard with responsive srcset).

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

backend/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.124.24"
+version = "0.125.0"
 edition = "2021"
 default-run = "mangalord"
 
@@ -62,6 +62,7 @@ http-body-util = "0.1"
 mime = "0.3"
 futures-util = "0.3"
 tokio = { version = "1", features = ["test-util"] }
+image = { version = "0.25", default-features = false, features = ["jpeg", "png", "webp"] }
 
 # Trim debug builds: keep line numbers in panics / backtraces but drop the
 # full DWARF info (variable-level inspection in gdb/lldb). With a sqlx +