feat: canonical intent taxonomy for AI operator (kb_answer -> intent_code)

Centralizes fixed control intents and adds a data-driven intent_code
field on kb_articles so many phrasings of the same FAQ question
resolve to one stable code (e.g. VOUCHER_ACTIVATION) instead of a
free-form, unvalidated string the LLM invented on the fly.

- services/shared/intents.py: CONTROL_INTENTS + normalize_intent()
- kb_articles.intent_code column (ORM + dev/sqlite runtime compat +
  migrations/sql/0034_* for postgres/sqlite)
- kb_service CRUD exposes intent_code
- orchestrator surfaces intent_code to the LLM and validates its
  intent output against control intents + the KB codes shown that turn
- voice.py: _voice_early_intent_bucket renamed to _voice_ack_topic_bucket
  to stop it being conflated with the canonical FAQ intent

services/shared/kb_search.py

@@ -25,6 +25,7 @@ class KBSearchRow(Protocol):
     title: str
     body: str
     tags_json: str
+    intent_code: str | None
 
 
 T = TypeVar("T", bound=KBSearchRow)