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 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.

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/eviction_internal_test.go

@@ -128,7 +128,7 @@ func storeEvictionTestVariant(
 ) {
 	t.Helper()
 
-	err := cache.StoreVariant(t.Context(), key, bytes.NewReader(content), "image/webp")
+	err := cache.StoreVariant(t.Context(), key, bytes.NewReader(content))
 	if err != nil {
 		t.Fatalf("StoreVariant(%s) failed: %v", key, err)
 	}
@@ -437,7 +437,7 @@ func assertDisabledCacheWritesAreNoOps(
 	ctx := t.Context()
 
 	err := cache.StoreVariant(
-		ctx, imagerequest.CacheKey(req), bytes.NewReader([]byte("data")), "image/webp",
+		ctx, imagerequest.CacheKey(req), bytes.NewReader([]byte("data")),
 	)
 	if err != nil {
 		t.Fatalf("StoreVariant on disabled cache must be a no-op, got error: %v", err)
@@ -667,9 +667,8 @@ func TestStartEvictionReconcilesAccountingWithDisk(t *testing.T) {
 
 	// An accounting row whose file is missing must be dropped.
 	_, err = cache.db.ExecContext(t.Context(),
-		`INSERT INTO variant_content (cache_key, size_bytes, content_type)
-		 VALUES (?, ?, ?)`,
-		"deadbeef0001", 700, "image/webp",
+		`INSERT INTO variant_content (cache_key, size_bytes) VALUES (?, ?)`,
+		"deadbeef0001", 700,
 	)
 	if err != nil {
 		t.Fatalf("failed to insert stale variant accounting row: %v", err)