refactor(scenario): only 2 scenarios (social / face-to-face); 're-contact' becomes a persona trait

- Scenario picker now has only social + f2f_call (removed recontact) in backend
  _scenarios, start_session validation, and Chat.vue picker.
- 'ลูกค้ากลับมาติดต่อ' is no longer a scenario: it's now a PERSONA trait. Persona prompt
  generates ~1-in-4 personas with recontact=true (asked before, now returns warmer/ready);
  store shape gets recontact field; simulator injects a re-contact note into the persona's
  system prompt so it plays as a returning customer naturally.
- test_scenario updated: unknown scenario defaults to social; recontact shown as a
  generated persona trait.
All 9 backend suites pass. Rebuilt dist.

backend/app/services/persona_prompts.py

@@ -25,6 +25,10 @@ EACH persona MUST include ALL of these fields:
 - tolerance (1-5): how many irritant/poor answers you tolerate before you walk away ("heart"). IMPORTANT:
   a temperamental/impatient persona has LOW tolerance (1-2, walks away fast after poor answers); a patient
   one has HIGH (4-5). Avg is 3. Match tolerance to personality (e.g. a busy owner / abrupt personality = low).
+- recontact (true/false): if true, this persona asked about the product BEFORE (earlier contact, e.g. a few
+  weeks/months back) and is ONLY NOW coming back / re-opening, more ready to buy and less price-sensitive.
+  These are already-pre-qualified, warmer leads. Make about 1 in every 4 personas recontact=true, spread across tiers.
+  A recontact persona opener/small-talk often references "I asked about this before" naturally.
 
 RULES:
 1. DIVERSITY: 15 distinct people across age groups, occupations, incomes, lifestyles,