Remove the code nothing in production calls (closes #312)

Service.Warm, the ImageCache interface, both GenerateSignedURL methods,
signature.ParseParams, imagerequest.ParseImagePath, four symbols of
internal/magic, the image processor's lists of supported formats,
IsEmpty and Count of the allowlist, ssrfSafeDialer, CacheStale,
ErrCacheMiss, ErrNegativeCache, ImageResponse.LastModified and the Load
methods of ContentStorage and VariantStorage had no caller outside
tests. They are gone, with the tests that tested only them.

The handlers tests sign their image URLs with generateSignedURL, a test
helper that builds the URL as the removed generator did. The SSRF dialer
tests call dialSSRFSafe with no operator blocklist, which is all
ssrfSafeDialer did. The tests of ParseImagePath and ContentStorage.Load
call ParseImageURL and LoadWithSize instead. MIMEToImageFormat,
ContentStorage.Store and CleanExpired stay, as pixa calls them.

Model: opus-5-5

TODO.md

@@ -30,6 +30,22 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-26 the code nothing in production called is gone (closes #312). The
+  handlers tests sign their image URLs with `generateSignedURL`, a test helper
+  that writes them as the removed generator did. Removed:
+    - `Service.Warm` and the `ImageCache` interface
+    - `Service.GenerateSignedURL` and `Signer.GenerateSignedURL`
+    - `signature.ParseParams`
+    - `imagerequest.ParseImagePath`
+    - `PeekAndValidate`, `IsSupportedMIMEType`, `ImageFormatToMIME` and
+      `FormatOriginal` of `internal/magic`
+    - `SupportedInputFormats` and `SupportedOutputFormats` of the image
+      processor
+    - `IsEmpty` and `Count` of the allowlist
+    - `ssrfSafeDialer`
+    - `CacheStale`, `ErrCacheMiss`, `ErrNegativeCache` and
+      `ImageResponse.LastModified`
+    - `ContentStorage.Load` and `VariantStorage.Load`
 - 2026-10-26 cache hits, misses, upstream fetches, the bytes those fetches read
   and transforms are counted as Prometheus counters in memory (closes #311).
   They are `pixa_cache_hits_total`, `pixa_cache_misses_total`,