Remove unimplemented Purge and three unused interfaces (closes #73)

Purge only returned an error and nothing called it, so it is gone from
the ImageCache interface and from Service, along with its error value.
It can be added back when something needs it; eviction reclaims disk
space meanwhile. The SignatureValidator, Allowlist and Storage
interfaces in imgcache.go had no implementers and no users, and
described types that look different from the real ones, so they are
deleted. No behaviour changes.

Model: opus-5-5

internal/imgcache/imgcache.go

@@ -5,7 +5,6 @@ import (
 	"context"
 	"errors"
 	"io"
-	"net/url"
 	"time"
 )
 
@@ -154,9 +153,6 @@ type ImageCache interface {
 	// Warm pre-fetches and caches an image without returning it
 	Warm(ctx context.Context, req *ImageRequest) error
 
-	// Purge removes a cached image
-	Purge(ctx context.Context, req *ImageRequest) error
-
 	// Stats returns cache statistics
 	Stats(ctx context.Context) (*CacheStats, error)
 }
@@ -176,29 +172,3 @@ type CacheStats struct {
 	// HitRate is HitCount / (HitCount + MissCount)
 	HitRate float64
 }
-
-// SignatureValidator validates request signatures
-type SignatureValidator interface {
-	// Validate checks if the signature is valid for the request
-	Validate(req *ImageRequest) error
-	// Generate creates a signature for a request
-	Generate(req *ImageRequest) string
-}
-
-// Allowlist checks if a URL is allowlisted (no signature required)
-type Allowlist interface {
-	// IsAllowlisted returns true if the URL doesn't require a signature
-	IsAllowlisted(u *url.URL) bool
-}
-
-// Storage handles persistent storage of cached content
-type Storage interface {
-	// Store saves content and returns its hash
-	Store(ctx context.Context, content io.Reader) (hash string, err error)
-	// Load retrieves content by hash
-	Load(ctx context.Context, hash string) (io.ReadCloser, error)
-	// Delete removes content by hash
-	Delete(ctx context.Context, hash string) error
-	// Exists checks if content exists
-	Exists(ctx context.Context, hash string) (bool, error)
-}