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/pendingcounts_internal_test.go

@@ -364,7 +364,7 @@ func TestLookupSource_HandsAccessTimeToTheBackgroundWriter(t *testing.T) {
 	`
 	anHourAgoInDatabase := anHourAgo.UTC().Format(sqliteTimestampFormat)
 
-	found, _, err := cache.LookupSource(t.Context(), &imagerequest.Request{
+	found, err := cache.LookupSource(t.Context(), &imagerequest.Request{
 		Source: imagerequest.Source{Host: "example.com", Path: "/photo.jpg"},
 	})
 	if err != nil || found != key {