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

internal/imgcache/imgcache.go

@@ -1,13 +1,7 @@
-// Package imgcache provides interfaces and types for the image caching proxy.
+// Package imgcache provides types for the image caching proxy.
 package imgcache
 
-import (
-	"context"
-	"io"
-	"time"
-
-	"sneak.berlin/go/pixa/internal/imagerequest"
-)
+import "io"
 
 // ImageResponse represents a processed image ready to serve
 type ImageResponse struct {
@@ -19,9 +13,7 @@ type ImageResponse struct {
 	ContentType string
 	// ETag is the entity tag for caching
 	ETag string
-	// LastModified is when the content was last modified
-	LastModified time.Time
-	// CacheStatus indicates HIT, MISS, or STALE
+	// CacheStatus indicates HIT or MISS
 	CacheStatus CacheStatus
 	// FetchedBytes is the number of bytes fetched from upstream (0 if cache hit)
 	FetchedBytes int64
@@ -32,16 +24,6 @@ type CacheStatus string
 
 // Cache status values for response headers.
 const (
-	CacheHit   CacheStatus = "HIT"
-	CacheMiss  CacheStatus = "MISS"
-	CacheStale CacheStatus = "STALE"
+	CacheHit  CacheStatus = "HIT"
+	CacheMiss CacheStatus = "MISS"
 )
-
-// ImageCache is the main interface for the image caching proxy
-type ImageCache interface {
-	// Get retrieves a processed image, fetching and processing if necessary
-	Get(ctx context.Context, req *imagerequest.Request) (*ImageResponse, error)
-
-	// Warm pre-fetches and caches an image without returning it
-	Warm(ctx context.Context, req *imagerequest.Request) error
-}