refactor: propagate context into Cache.StoreVariant (noctx)

The size-accounting INSERT in StoreVariant used db.Exec, and
StoreVariant took no context at all, so the write was uncancellable and
untraceable. Rather than paper over that with context.Background() at
the call site, StoreVariant now takes a context.Context and uses
ExecContext.

The plumbing is small: the only production caller is
Service.processAndStore, which already has the request context in
scope, so the accounting insert now shares the lifetime of the request
that produced the variant. Test callers pass t.Context().

The insert remains best-effort: a failure is logged and the startup and
periodic reconciliation passes still adopt any variant file whose
accounting row is missing.

internal/imgcache/cache.go

@@ -291,7 +291,7 @@ func (c *Cache) StoreSource(
 // accounting insert is best-effort (the startup reconciliation pass
 // adopts any variant file that misses its accounting row).
 func (c *Cache) StoreVariant(
-	cacheKey VariantKey, content io.Reader, contentType string,
+	ctx context.Context, cacheKey VariantKey, content io.Reader, contentType string,
 ) error {
 	if c.disabled {
 		return nil
@@ -302,7 +302,7 @@ func (c *Cache) StoreVariant(
 		return err
 	}
 
-	_, err = c.db.Exec(`
+	_, err = c.db.ExecContext(ctx, `
 		INSERT INTO variant_content (cache_key, size_bytes, content_type)
 		VALUES (?, ?, ?)
 		ON CONFLICT(cache_key) DO UPDATE SET