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

migrations/sql/0034_kb_intent_code_postgres.sql

@@ -0,0 +1,2 @@
+ALTER TABLE kb_articles ADD COLUMN IF NOT EXISTS intent_code TEXT;
+CREATE INDEX IF NOT EXISTS idx_kb_articles_intent_code ON kb_articles(intent_code);

migrations/sql/0034_kb_intent_code_sqlite.sql

@@ -0,0 +1,2 @@
+ALTER TABLE kb_articles ADD COLUMN intent_code TEXT;
+CREATE INDEX IF NOT EXISTS idx_kb_articles_intent_code ON kb_articles(intent_code);