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.go

@@ -297,17 +297,18 @@ func (c *Cache) LookupSource(
 
 	key := req.Source.Key()
 
-	var rowExists bool
+	var foundKey string
 
 	err := c.db.QueryRowContext(ctx, `
-		SELECT EXISTS(SELECT 1 FROM source_content WHERE source_key = ?)
-	`, string(key)).Scan(&rowExists)
-	if err != nil {
-		return "", fmt.Errorf("failed to lookup source: %w", err)
+		SELECT source_key FROM source_content WHERE source_key = ?
+	`, string(key)).Scan(&foundKey)
+
+	if errors.Is(err, sql.ErrNoRows) {
+		return "", nil
 	}
 
-	if !rowExists {
-		return "", nil
+	if err != nil {
+		return "", fmt.Errorf("failed to lookup source: %w", err)
 	}
 
 	// Verify the content file exists