Fix: pg_trgm optional, table creation no longer fails without it

Trigram extension and index are now created in a separate try/catch
so table creation succeeds even without pg_trgm. Search falls back
to ILIKE when trigram functions are not available.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend-lehrer/vocabulary_db.py

@@ -75,14 +75,16 @@ async def init_vocabulary_tables():
                 ON vocabulary_words (difficulty);
             CREATE INDEX IF NOT EXISTS idx_vocab_tags
                 ON vocabulary_words USING GIN (tags);
+        """)
+        # Enable trigram extension for fuzzy search (optional)
+        try:
+            await conn.execute("CREATE EXTENSION IF NOT EXISTS pg_trgm;")
+            await conn.execute("""
                 CREATE INDEX IF NOT EXISTS idx_vocab_english_trgm
                     ON vocabulary_words USING GIN (english gin_trgm_ops);
             """)
-        # Enable trigram extension for fuzzy search (may already exist)
-        try:
-            await conn.execute("CREATE EXTENSION IF NOT EXISTS pg_trgm;")
         except Exception:
-            logger.info("pg_trgm extension already exists or cannot be created")
+            logger.info("pg_trgm not available — trigram search disabled, using LIKE fallback")
 
     logger.info("vocabulary_words table initialized")
 
@@ -140,10 +142,12 @@ def _row_to_word(row) -> VocabularyWord:
 async def search_words(
     query: str, lang: str = "en", limit: int = 20, offset: int = 0,
 ) -> List[VocabularyWord]:
-    """Full-text search for words."""
+    """Full-text search for words. Uses trigram similarity if available, else ILIKE."""
     pool = await get_pool()
     col = "english" if lang == "en" else "german"
     async with pool.acquire() as conn:
+        # Try trigram search first, fall back to ILIKE
+        try:
             rows = await conn.fetch(
                 f"""
                 SELECT * FROM vocabulary_words
@@ -153,6 +157,16 @@ async def search_words(
                 """,
                 f"%{query.lower()}%", query, limit, offset,
             )
+        except Exception:
+            rows = await conn.fetch(
+                f"""
+                SELECT * FROM vocabulary_words
+                WHERE lower({col}) LIKE $1
+                ORDER BY lower({col})
+                LIMIT $2 OFFSET $3
+                """,
+                f"%{query.lower()}%", limit, offset,
+            )
     return [_row_to_word(r) for r in rows]