Declare only the tables and columns pixa writes and reads (closes #310)

Removes output_content and request_cache, which nothing writes, and every
column that was written but never read: the host, path, query, content_type
and fetched_at of source_content, the content_type and created_at of
variant_content, and the status_code, error_message and fetched_at of
negative_cache.

StoreSource no longer takes the fetch result and LookupSource returns only
the source key. StoreVariant no longer takes a content type and
StoreNegative no longer takes a status code or message, so
extractStatusCode and the status constants, which only fed status_code,
are gone. Reconciliation adopts a variant file without opening it, as its
row no longer records a content type. Tests that set or checked a removed
column follow.

Model: opus-5-5

internal/imgcache/cache_hit_internal_test.go

@@ -16,9 +16,8 @@ import (
 )
 
 // TestGetVariant_ContentTypeFromFirstBytes stores an image in each output
-// format as a variant, recorded with the content type text/plain, and checks
-// that GetVariant serves it whole with the content type of its format, read
-// from the file.
+// format as a variant and checks that GetVariant serves it whole with the
+// content type of its format, read from the file.
 func TestGetVariant_ContentTypeFromFirstBytes(t *testing.T) {
 	t.Parallel()
 
@@ -48,7 +47,7 @@ func TestGetVariant_ContentTypeFromFirstBytes(t *testing.T) {
 
 		key := imagerequest.VariantKey("aabbccdd10" + string(format))
 
-		err = cache.StoreVariant(t.Context(), key, bytes.NewReader(image), "text/plain")
+		err = cache.StoreVariant(t.Context(), key, bytes.NewReader(image))
 		if err != nil {
 			t.Fatalf("StoreVariant(%s) error = %v", format, err)
 		}