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/sql_init.py

@@ -583,6 +583,7 @@ def _apply_runtime_schema_compatibility() -> None:
         if "kb_articles" in table_names:
             columns = _table_columns(inspector, "kb_articles")
             _add_column_if_missing(conn, columns, "kb_articles", "article_group_id", "VARCHAR(64)")
+            _add_column_if_missing(conn, columns, "kb_articles", "intent_code", "VARCHAR(64)")
             _add_column_if_missing(conn, columns, "kb_articles", "language", "VARCHAR(8) DEFAULT 'ru'")
             if "language" in columns:
                 conn.execute(
@@ -619,6 +620,13 @@ def _apply_runtime_schema_compatibility() -> None:
                         "ON kb_articles(language)"
                     )
                 )
+            if "idx_kb_articles_intent_code" not in indexes:
+                conn.execute(
+                    text(
+                        "CREATE INDEX IF NOT EXISTS idx_kb_articles_intent_code "
+                        "ON kb_articles(intent_code)"
+                    )
+                )
 
         if "sales_automation_tasks" in table_names:
             columns = _table_columns(inspector, "sales_automation_tasks")