Activate Captain AI features: config, RAG routes, conversation handler

Phase 1 of AI_FEATURE_ROADMAP.md:

1. config.yaml: Add captain configuration section (llm_provider, llm_model,
   llm_api_key, llm_base_url, embedding_model, embedding_dims, max_tokens,
   temperature). Secrets injected via GOCHAT_CAPTAIN_LLM_API_KEY env var.

2. RAG route registration:
   - bootstrap.go: instantiate RAGService + RAGHandler, add to Handlers
   - router.go: register POST /captain/rag/query + /captain/rag/index/:response_id
   - rag_handler.go: fix account_id param name (was "id", route uses "account_id")

3. CaptainConversationService activation:
   - Remove "_ = captainConversationService" ignore in bootstrap.go
   - Create CaptainConversationHandler with BuildResponse endpoint
   - Register POST /captain/conversations/:conversation_id/respond route
   - Fix account_id param name in handler

Verified:
- go build ./... passes
- go vet passes on all modified packages
- go test passes (llm + service packages)
- Server starts with captain config loaded
- RAG query/index routes return proper handler responses (not 404)
- Conversation respond route returns proper skip for non-pending conversations
- Existing captain routes unaffected (assistants list still works)

backend/internal/handler/api/v1/captain_conversation_handler.go

@@ -0,0 +1,67 @@
+package v1
+
+import (
+	"net/http"
+	"strconv"
+
+	"github.com/gin-gonic/gin"
+	"github.com/gochat/gochat/internal/service"
+	applogger "github.com/gochat/gochat/pkg/logger"
+	"github.com/gochat/gochat/pkg/response"
+)
+
+// CaptainConversationHandler handles Captain conversation-level AI auto-response.
+// Reference: M12 PRD §Captain AI — Conversation Auto-Response (handoff mode)
+//
+// This handler exposes the CaptainConversationService, which generates AI
+// responses for conversations bound to a Captain Assistant. When the AI
+// determines a handoff is needed, the conversation is transferred to a human
+// agent instead of sending an AI-generated reply.
+type CaptainConversationHandler struct {
+	svc *service.CaptainConversationService
+}
+
+// NewCaptainConversationHandler creates a new CaptainConversationHandler.
+func NewCaptainConversationHandler(svc *service.CaptainConversationService) *CaptainConversationHandler {
+	return &CaptainConversationHandler{svc: svc}
+}
+
+// BuildResponse triggers AI response generation for a conversation.
+// The assistant must be bound to the conversation's inbox via CaptainInbox.
+//
+// POST /api/v1/accounts/:account_id/captain/conversations/:conversation_id/respond
+func (h *CaptainConversationHandler) BuildResponse(c *gin.Context) {
+	accountID, err := strconv.ParseUint(c.Param("account_id"), 10, 64)
+	if err != nil {
+		response.AbortWithStatusError(c, http.StatusBadRequest, "invalid_account_id", "Invalid account ID")
+		return
+	}
+
+	conversationID, err := strconv.ParseUint(c.Param("conversation_id"), 10, 64)
+	if err != nil {
+		response.AbortWithStatusError(c, http.StatusBadRequest, "invalid_conversation_id", "Invalid conversation ID")
+		return
+	}
+
+	var req struct {
+		AssistantID uint `json:"assistant_id" binding:"required"`
+	}
+	if err := c.ShouldBindJSON(&req); err != nil {
+		response.AbortWithStatusError(c, http.StatusBadRequest, "invalid_request", err.Error())
+		return
+	}
+
+	msg, err := h.svc.BuildConversationResponseByAccount(c.Request.Context(), uint(accountID), uint(conversationID), req.AssistantID)
+	if err != nil {
+		applogger.L().Errorf("CaptainConversationHandler.BuildResponse: %v", err)
+		response.AbortWithStatusError(c, http.StatusInternalServerError, "response_failed", "Failed to generate AI response")
+		return
+	}
+
+	if msg == nil {
+		response.OK(c, map[string]any{"status": "skipped", "message": "Conversation not eligible for AI response"})
+		return
+	}
+
+	response.OK(c, map[string]any{"status": "sent", "message_id": msg.ID})
+}

backend/internal/handler/api/v1/rag_handler.go

@@ -24,7 +24,7 @@ func NewRAGHandler(svc *service.RAGService) *RAGHandler {
 // Query performs a RAG query against the knowledge base.
 // POST /api/v1/accounts/:account_id/captain/rag/query
 func (h *RAGHandler) Query(c *gin.Context) {
-	accountID, err := strconv.ParseUint(c.Param("id"), 10, 64)
+	accountID, err := strconv.ParseUint(c.Param("account_id"), 10, 64)
 	if err != nil {
 		response.AbortWithStatusError(c, http.StatusBadRequest, "invalid_account_id", "Invalid account ID")
 		return