Add Phase 2+3 features: antonyms, backing, pronunciation, etymology, difficulty, affix expansion

New endpoints: /antonyms, /backing, /pronunciation, /etymology with difficulty
scoring on /random. Cross-language synset backing links French/Portuguese words
to English equivalents via WordNet 3.0 synset IDs (matching WOLF and OMW offsets).

New loaders: Hunspell affix expansion (fr, pt-PT), English programmatic inflector,
CMU Pronouncing Dictionary, SUBTLEX-US frequency, CETEMPúblico frequency,
Open Multilingual Wordnet (Portuguese), and SQL-based difficulty scoring.

Schema v2 adds tables: antonyms, synsets, word_synsets, pronunciations, etymology
with migration support for existing databases.

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

internal/loader/scowl.go

@@ -68,7 +68,8 @@ func (SCOWLLoader) Load(db *sql.DB, dataDir string) error {
 	}
 	defer tx.Rollback()
 
-	stmt, err := tx.Prepare("INSERT OR IGNORE INTO words (word, lang, frequency) VALUES (?, 'en', ?)")
+	stmt, err := tx.Prepare(`INSERT INTO words (word, lang, frequency) VALUES (?, 'en', ?)
+		ON CONFLICT(word, lang) DO UPDATE SET frequency = MAX(frequency, excluded.frequency)`)
 	if err != nil {
 		return fmt.Errorf("scowl: prepare: %w", err)
 	}