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/testutil_internal_test.go

@@ -20,12 +20,10 @@ import (
 
 // Shared test data literals, extracted as constants for goconst.
 const (
-	testHostCDN           = "cdn.example.com"
-	testHostExample       = "example.com"
-	testPathCat           = "/photos/cat.jpg"
-	testContentTypeJPEG   = "image/jpeg"
-	testContentTypeWebP   = "image/webp"
-	testHeaderContentType = "Content-Type"
+	testHostCDN         = "cdn.example.com"
+	testHostExample     = "example.com"
+	testPathCat         = "/photos/cat.jpg"
+	testContentTypeJPEG = "image/jpeg"
 )
 
 // TestFixtures contains paths to test files in the mock filesystem.