feat(perf): implement HTTP connection pooling with httpx.AsyncClient singleton and Keep-Alive

- get_http_client: singleton AsyncClient with limits(max_keepalive=10, max_connections=20, keepalive_expiry=60s)
- close_http_client: clean socket pool teardown in FastAPI lifespan shutdown
- refactor dictionary fetchers to reuse pooled client and avoid 5x TLS handshakes per word lookup
- add test_http_connection_pooling (22/22 pytest passed)

app/main.py

@@ -25,7 +25,7 @@ from app.auth import (
     record_login_success,
     change_user_password,
 )
-from app.dictionary import lookup_term, USER_AGENT
+from app.dictionary import lookup_term, USER_AGENT, close_http_client
 from app.export import export_csv, export_json, export_anki_tsv, export_markdown
 from app.srs import calculate_sm2, generate_quiz_options, build_activity_heatmap, calculate_cefr_level
 from app.reader import parse_epub, parse_text, parse_pdf, count_words
@@ -36,6 +36,7 @@ async def lifespan(app: FastAPI):
     init_db()
     ensure_admin_user()
     yield
+    await close_http_client()
 
 
 app = FastAPI(