Add regional variant tagging (us/gb) for English words

- SCOWL loader now loads american-words and british-words alongside
  english-words, tagging each with variant "us", "gb", or NULL (common)
- Words appearing in both American and British lists get NULL variant
- Add variant column to words table with schema migration
- API: /define returns variant field for English, /random and /words
  accept ?variant=us|gb filter
- Add total word count to server status line
- Fix rows.Close leak in PopulateCMUTails, add rows.Err() check
- Add tests for PopulateCMUTails, cmuTail, TotalWordCount
- Expand test seed data and assertions to cover pt-PT definitions,
  synonyms, and validity checks
- Update README with variant docs, /difficulty, /words, /rhyme
  endpoints, and current database statistics

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/dictionary/dict.go

@@ -28,6 +28,7 @@ type Options struct {
 	MinFrequency  int     // 0 = no filter
 	MinDifficulty float64 // 0.0 = no filter
 	MaxDifficulty float64 // 0.0 = no filter
+	Variant       string  // "", "us", "gb" — empty means no filter
 }
 
 type Definition struct {