Check cached sources with upstream after the cache TTL (closes #69)

Cached images were served until eviction, so upstream changes were
missed.

After 24 hours a request checks the source with upstream, sending its
stored ETag and Last-Modified. A 304 renews it with no download or
conversion. New bytes replace it; older variants are converted again.
A failed check serves the cached image and puts the URL in the
negative cache. Images not checked for 30 days are evicted.

Variant rows keep the source's ETag and Last-Modified for checking a
variant of an evicted source.

A variant file's modification time is its source's check time when it
was made or renewed. Hits read it without a database call. The ETag
includes it, so new bytes get a new ETag.

Model: opus-5-5

internal/imgcache/eviction_internal_test.go

@@ -101,7 +101,7 @@ func storeEvictionTestSource(
 	}
 
 	key, err := cache.PutSource(
-		context.Background(), req.Source, bytes.NewReader(content),
+		context.Background(), req.Source, bytes.NewReader(content), SourceInfo{},
 	)
 	if err != nil {
 		t.Fatalf("PutSource(%s%s) failed: %v", host, path, err)
@@ -117,7 +117,8 @@ func storeEvictionTestVariant(
 ) {
 	t.Helper()
 
-	err := cache.PutVariant(t.Context(), key, bytes.NewReader(content))
+	err := cache.PutVariant(t.Context(), key, SourceInfo{CheckedAt: time.Now()},
+		bytes.NewReader(content))
 	if err != nil {
 		t.Fatalf("PutVariant(%s) failed: %v", key, err)
 	}
@@ -432,13 +433,14 @@ func assertDisabledCacheWritesAreNoOps(
 	ctx := t.Context()
 
 	err := cache.PutVariant(
-		ctx, imagerequest.CacheKey(req), bytes.NewReader([]byte("data")),
+		ctx, imagerequest.CacheKey(req), SourceInfo{}, bytes.NewReader([]byte("data")),
 	)
 	if err != nil {
 		t.Fatalf("PutVariant on disabled cache must be a no-op, got error: %v", err)
 	}
 
-	key, err := cache.PutSource(ctx, req.Source, bytes.NewReader([]byte("data")))
+	key, err := cache.PutSource(ctx, req.Source, bytes.NewReader([]byte("data")),
+		SourceInfo{})
 	if err != nil {
 		t.Fatalf("PutSource on disabled cache must be a no-op, got error: %v", err)
 	}