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/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