Add word frequency endpoint and SCOWL frequency data

SCOWL loader now sets frequency based on tier (10=1000 most common,
70=50 rare). New GET /frequency?word=X&lang=Y endpoint returns frequency
score for a word. Enables downstream consumers to identify rare/
sophisticated vocabulary from dictionary data rather than LLM guessing.

Also fix .gitignore matching cmd/server/ directory instead of just the
server binary.

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

.gitignore

@@ -1,5 +1,5 @@
 dict.db
 data/
 !data/.gitkeep
-dictimport
-server
+/dictimport
+/server