Stop cache eviction in progress at shutdown (closes #102)

StartEviction runs the eviction goroutine with its own context, which
StopEviction cancels in place of the old stop channel, so a pass in
progress stops at its next database call, file, row or eviction
candidate instead of running to completion, and no new pass starts.
StopEviction takes a context: when it ends before the goroutine exits,
StopEviction stops waiting and returns an error wrapping it. The
handlers' stop hook passes fx's stop context, so an eviction still
running at fx's stop deadline fails the stop and the exit code is 1.
A stop logs at most one warning.

Model: opus-5-5

internal/imgcache/cache.go

@@ -11,7 +11,6 @@ import (
 	"io"
 	"log/slog"
 	"path/filepath"
-	"sync"
 	"time"
 
 	lru "github.com/hashicorp/golang-lru/v2"
@@ -69,11 +68,11 @@ type Cache struct {
 
 	// Eviction machinery. The channels are created in NewCache so
 	// stores can signal write pressure without racing StartEviction.
+	// evictionCancel, set by StartEviction, cancels the eviction
+	// goroutine's context.
 	evictionPressure chan struct{}
-	evictionStop     chan struct{}
 	evictionDone     chan struct{}
-	evictionStarted  bool
-	evictionStopOnce sync.Once
+	evictionCancel   context.CancelFunc
 
 	// metaCache holds the content types of the variants most recently
 	// stored or served, so a hit does not read the variant's .meta file.
@@ -112,7 +111,6 @@ func NewCache(db *sql.DB, config CacheConfig) (*Cache, error) {
 		log:              log,
 		disabled:         config.DisableDiskCache,
 		evictionPressure: make(chan struct{}, 1),
-		evictionStop:     make(chan struct{}),
 		evictionDone:     make(chan struct{}),
 		metaCache:        metaCache,
 		contentLocks:     newContentLock(),