feat: integrate bettervent.me device database and add BTU/kW unit switcher

Add bettervent.me provider with lazy-cached device list (~6,700 Eurovent-certified
heat pumps), search API endpoint, device search UI with auto-populate, BTU/kW unit
switcher for European users, and extended AC fields (SEER, SCOP, COP, TOL, Tbiv,
refrigerant). Closes #2.

internal/server/api.go

@@ -311,6 +311,25 @@ func (s *Server) handleLLMConfig(w http.ResponseWriter, r *http.Request) {
 	})
 }
 
+func (s *Server) handleBetterventSearch(w http.ResponseWriter, r *http.Request) {
+	if r.Method != http.MethodGet {
+		http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
+		return
+	}
+
+	q := r.URL.Query().Get("q")
+	ctx, cancel := context.WithTimeout(r.Context(), 30*time.Second)
+	defer cancel()
+
+	resp, err := s.betterventProvider.Search(ctx, q)
+	if err != nil {
+		jsonError(w, err.Error(), http.StatusBadGateway)
+		return
+	}
+
+	jsonResponse(w, resp)
+}
+
 func jsonResponse(w http.ResponseWriter, data any) {
 	w.Header().Set("Content-Type", "application/json")
 	json.NewEncoder(w).Encode(data)

internal/server/server.go

@@ -10,18 +10,20 @@ import (
 	"path/filepath"
 	"sync/atomic"
 
+	"github.com/cnachtigall/heatwave-autopilot/internal/bettervent"
 	"github.com/cnachtigall/heatwave-autopilot/internal/config"
 	"github.com/cnachtigall/heatwave-autopilot/internal/llm"
 )
 
 // Server holds the HTTP server state.
 type Server struct {
-	mux         *http.ServeMux
-	trans       *translations
-	cfg         config.Config
-	llmProvider llm.Provider
-	devMode     bool
-	ready       atomic.Bool
+	mux                *http.ServeMux
+	trans              *translations
+	cfg                config.Config
+	llmProvider        llm.Provider
+	betterventProvider *bettervent.Provider
+	devMode            bool
+	ready              atomic.Bool
 }
 
 // Options configures the server.
@@ -73,6 +75,9 @@ func New(opts Options) (*Server, error) {
 	// Set up LLM provider
 	s.llmProvider = buildLLMProvider(s.cfg)
 
+	// Set up bettervent provider
+	s.betterventProvider = bettervent.New(nil)
+
 	// Static assets
 	if opts.DevMode {
 		s.mux.Handle("/assets/", http.StripPrefix("/assets/", http.FileServer(http.Dir("web"))))
@@ -96,6 +101,7 @@ func New(opts Options) (*Server, error) {
 	s.mux.HandleFunc("/api/llm/summarize", s.handleLLMSummarize)
 	s.mux.HandleFunc("/api/llm/actions", s.handleLLMActions)
 	s.mux.HandleFunc("/api/llm/config", s.handleLLMConfig)
+	s.mux.HandleFunc("/api/bettervent/search", s.handleBetterventSearch)
 
 	s.ready.Store(true)
 	return s, nil

internal/server/server_test.go

@@ -188,6 +188,46 @@ func TestReadyz(t *testing.T) {
 	}
 }
 
+func TestBetterventSearch(t *testing.T) {
+	s := testServer(t)
+	req := httptest.NewRequest("GET", "/api/bettervent/search?q=test", nil)
+	w := httptest.NewRecorder()
+	s.Handler().ServeHTTP(w, req)
+	// The test server won't reach bettervent.me, but the handler should still
+	// return valid JSON (either results or a 502 error).
+	if ct := w.Header().Get("Content-Type"); ct != "application/json" {
+		t.Errorf("got content-type %q, want application/json", ct)
+	}
+}
+
+func TestBetterventSearch_EmptyQuery(t *testing.T) {
+	s := testServer(t)
+	req := httptest.NewRequest("GET", "/api/bettervent/search?q=", nil)
+	w := httptest.NewRecorder()
+	s.Handler().ServeHTTP(w, req)
+	if w.Code != http.StatusOK {
+		t.Errorf("got status %d, want 200", w.Code)
+	}
+	var resp struct {
+		Devices []any `json:"devices"`
+		Total   int   `json:"total"`
+	}
+	json.NewDecoder(w.Body).Decode(&resp)
+	if resp.Total != 0 {
+		t.Errorf("expected 0 results for empty query, got %d", resp.Total)
+	}
+}
+
+func TestBetterventSearch_MethodNotAllowed(t *testing.T) {
+	s := testServer(t)
+	req := httptest.NewRequest("POST", "/api/bettervent/search?q=test", nil)
+	w := httptest.NewRecorder()
+	s.Handler().ServeHTTP(w, req)
+	if w.Code != http.StatusMethodNotAllowed {
+		t.Errorf("got status %d, want 405", w.Code)
+	}
+}
+
 func TestLLMSummarize_Noop(t *testing.T) {
 	s := testServer(t)
 	body := `{"date":"2025-07-15","peakTempC":35,"riskLevel":"high"}`