test: add e2e tests for all 9 email handlers (54 tests)

Mock send_email and call each handler directly. Covers recipient,
subject content, HTML design elements (wordmark, preheader, heat
badges), from_addr, skip-on-missing-data guards, and email_sent_at
timestamp updates.

Also fixes IndexError in handle_send_welcome when payload has no name
("".split()[0] → safe fallback).

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

web/src/padelnomics/worker.py

@@ -298,7 +298,8 @@ async def handle_send_quote_verification(payload: dict) -> None:
 async def handle_send_welcome(payload: dict) -> None:
     """Send welcome email to new user."""
     lang = payload.get("lang", "en")
-    first_name = (payload.get("name") or "").split()[0] or "there"
+    name_parts = (payload.get("name") or "").split()
+    first_name = name_parts[0] if name_parts else "there"
 
     body = (
         f'<h2 style="margin:0 0 8px;color:#0F172A;font-size:22px;">{_t("email_welcome_heading", lang, app_name=config.APP_NAME)}</h2>'