feat(chatbot): OSS self-contained guardrail + knowledge-base chatbot

Built-in AI chatbot (no EE, uses Llm::Resolver) that answers in-scope chats
from a knowledge base and hands off to a human when needed. Selected per
inbox via an Integrations::Hook with app_id 'chatbot'.

- Integrations::Chatbot::ProcessorService (mirrors Dialogflow/Captain)
  wired via HookListener + HookJob + apps.yml(chatbot, inbox)
- Chatbot::DecisionService: 1-call default ({in_scope/refuse/handoff}),
  2-call option; thread-safe prompt threading
- Chatbot::KnowledgeRetriever: keyword top-k over KB (+ embedding reserved)
- KnowledgeBaseFaq + import service (md per-heading + front-matter tags)
- Chatbot::ConfigService + admin chatbot_config endpoint
- refs off-topic (e.g. fortune-telling); handoff = bot_handoff! (pending->open)

app/models/knowledge_base_faq.rb

@@ -0,0 +1,34 @@
+# == Schema Information
+#
+# Table name: knowledge_base_faqs
+#
+#  id              :bigint           not null, primary key
+#  account_id      :bigint           not null
+#  title           :string           not null
+#  content         :text             not null
+#  topic_tags      :jsonb            default: []       not null
+#  source_filename :string
+#  embedding       :vector(1536)
+#  created_at      :datetime         not null
+#  updated_at      :datetime         not null
+#
+class KnowledgeBaseFaq < ApplicationRecord
+  belongs_to :account
+  # pgvector KNN support (matches repo pattern: has_neighbors + nearest_neighbors)
+  has_neighbors :embedding, normalize: true
+
+  validates :title, presence: true
+  validates :content, presence: true
+
+  # topic_tags stored as jsonb array; expose string-list helpers for import/retrieval.
+  def topic_tag_list
+    Array(topic_tags)
+  end
+
+  def topic_tag_list=(value)
+    self.topic_tags = Array(value).map(&:strip).reject(&:blank?)
+  end
+
+  scope :for_account, ->(account) { where(account_id: account.id) }
+  scope :with_embedding, -> { where.not(embedding: nil) }
+end