refactor: extract queryStringColumn helper for the accounting queries (dupl)

allVariantKeys and allSourceContentHashes were byte-for-byte the same
query-scan-collect loop over a single string column, differing only in
the SQL, the result type and the noun in their error messages. Both now
delegate to a generic queryStringColumn helper.

Extracted rather than suppressed with a nolint: the duplication was
real, and the two call sites are the only places this shape appears.
Every error message is preserved verbatim, which is why the helper takes
both a plural and a singular noun.

internal/imgcache/eviction.go

@@ -2,6 +2,7 @@ package imgcache
 
 import (
 	"context"
+	"database/sql"
 	"encoding/json"
 	"fmt"
 	"io/fs"
@@ -647,32 +648,43 @@ func (c *Cache) reconcileVariantRows(ctx context.Context) error {
 
 // allVariantKeys returns every tracked variant cache key.
 func (c *Cache) allVariantKeys(ctx context.Context) ([]VariantKey, error) {
-	rows, err := c.db.QueryContext(ctx, `SELECT cache_key FROM variant_content`)
+	return queryStringColumn[VariantKey](ctx, c.db,
+		`SELECT cache_key FROM variant_content`, "variant keys", "variant key")
+}
+
+// queryStringColumn runs a single-column query and returns the column
+// values as T. plural names the set for the query and scan failure
+// messages; singular names one row for the scan and iteration failure
+// messages.
+func queryStringColumn[T ~string](
+	ctx context.Context, db *sql.DB, query, plural, singular string,
+) ([]T, error) {
+	rows, err := db.QueryContext(ctx, query)
 	if err != nil {
-		return nil, fmt.Errorf("failed to query variant keys: %w", err)
+		return nil, fmt.Errorf("failed to query %s: %w", plural, err)
 	}
 
 	defer func() { _ = rows.Close() }()
 
-	var keys []VariantKey
+	var values []T
 
 	for rows.Next() {
-		var key string
+		var value string
 
-		err := rows.Scan(&key)
+		err := rows.Scan(&value)
 		if err != nil {
-			return nil, fmt.Errorf("failed to scan variant key: %w", err)
+			return nil, fmt.Errorf("failed to scan %s: %w", singular, err)
 		}
 
-		keys = append(keys, VariantKey(key))
+		values = append(values, T(value))
 	}
 
 	err = rows.Err()
 	if err != nil {
-		return nil, fmt.Errorf("variant key iteration failed: %w", err)
+		return nil, fmt.Errorf("%s iteration failed: %w", singular, err)
 	}
 
-	return keys, nil
+	return values, nil
 }
 
 // reconcileSourceFiles walks the source content directory, removing
@@ -763,32 +775,9 @@ func (c *Cache) reconcileSourceRows(ctx context.Context) error {
 
 // allSourceContentHashes returns every tracked source content hash.
 func (c *Cache) allSourceContentHashes(ctx context.Context) ([]ContentHash, error) {
-	rows, err := c.db.QueryContext(ctx, `SELECT content_hash FROM source_content`)
-	if err != nil {
-		return nil, fmt.Errorf("failed to query source content hashes: %w", err)
-	}
-
-	defer func() { _ = rows.Close() }()
-
-	var hashes []ContentHash
-
-	for rows.Next() {
-		var hash string
-
-		err := rows.Scan(&hash)
-		if err != nil {
-			return nil, fmt.Errorf("failed to scan content hash: %w", err)
-		}
-
-		hashes = append(hashes, ContentHash(hash))
-	}
-
-	err = rows.Err()
-	if err != nil {
-		return nil, fmt.Errorf("content hash iteration failed: %w", err)
-	}
-
-	return hashes, nil
+	return queryStringColumn[ContentHash](ctx, c.db,
+		`SELECT content_hash FROM source_content`,
+		"source content hashes", "content hash")
 }
 
 // sweepStaleTempFile removes a temp file left behind by a crashed