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/dictionary/dict.go

@@ -22,10 +22,12 @@ func ValidLang(lang string) bool {
 }
 
 type Options struct {
-	MinLength    int
-	MaxLength    int
-	POS          string // "", "noun", "verb", "adjective", "adverb"
-	MinFrequency int    // 0 = no filter
+	MinLength     int
+	MaxLength     int
+	POS           string  // "", "noun", "verb", "adjective", "adverb"
+	MinFrequency  int     // 0 = no filter
+	MinDifficulty float64 // 0.0 = no filter
+	MaxDifficulty float64 // 0.0 = no filter
 }
 
 type Definition struct {
@@ -35,6 +37,18 @@ type Definition struct {
 	Priority int    `json:"priority"`
 }
 
+type EnglishEquivalent struct {
+	Word       string `json:"word"`
+	Definition string `json:"definition"`
+	Synset     string `json:"synset"`
+}
+
+type Pronunciation struct {
+	Format string `json:"format"`
+	Value  string `json:"value"`
+	Source string `json:"source"`
+}
+
 type Dictionary struct {
 	db *sql.DB
 }