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/max_concurrent_processing_internal_test.go

@@ -77,7 +77,7 @@ func TestService_Get_WaitsForSlotBeforeReadingCachedSource(t *testing.T) {
 
 	_ = resp.Content.Close()
 
-	sourceKey, _, err := svc.cache.LookupSource(t.Context(),
+	sourceKey, err := svc.cache.LookupSource(t.Context(),
 		widthOnlyRequest(fixtures, 50))
 	if err != nil || sourceKey == "" {
 		t.Fatalf("LookupSource() = %q, %v; want the cached source",