feat(proxy): add /auth/token and /auth/refresh endpoints (#18)

Closes homelab#6 (P3.1) and homelab#8 (P3.3)

## Endpoints

| Path | Method | Body | What it does |
|------|--------|------|-------------|
| `/auth/token` | POST | `{username, password, scope?}` | Password grant → JWT |
| `/auth/refresh` | POST | `{refresh_token, scope?}` | Refresh grant → new JWT |

Both proxy to Authentik `tokenUrl` (from P3.7 config). Upstream response forwarded verbatim — client sees Authentik errors directly.

internal/proxy/auth_endpoints.go

@@ -0,0 +1,160 @@
+package proxy
+
+import (
+	"encoding/json"
+	"io"
+	"net/http"
+	"net/url"
+	"time"
+
+	"forgejo.riotpiao.com/rock/homelab-frontend/internal/logging"
+)
+
+// tokenRequest is the JSON body for POST /auth/token.
+type tokenRequest struct {
+	Username string `json:"username"`
+	Password string `json:"password"`
+	Scope    string `json:"scope,omitempty"`
+}
+
+// refreshRequest is the JSON body for POST /auth/refresh.
+type refreshRequest struct {
+	RefreshToken string `json:"refresh_token"`
+	Scope        string `json:"scope,omitempty"`
+}
+
+// authClient handles token exchange with the upstream identity provider.
+// Extracted for testability — production uses http.DefaultClient,
+// tests inject a stub.
+type authClient interface {
+	PostForm(url string, data url.Values) (*http.Response, error)
+}
+
+// httpAuthClient wraps http.Client to implement authClient.
+type httpAuthClient struct {
+	client *http.Client
+}
+
+func (c *httpAuthClient) PostForm(url string, data url.Values) (*http.Response, error) {
+	return c.client.PostForm(url, data)
+}
+
+func newAuthClient() authClient {
+	return &httpAuthClient{
+		client: &http.Client{Timeout: 10 * time.Second},
+	}
+}
+
+// handleAuthToken exchanges username+password for a JWT via the upstream
+// identity provider's token endpoint using grant_type=password.
+func (h *Handler) handleAuthToken(w http.ResponseWriter, r *http.Request) {
+	if r.Method != "POST" {
+		writeProblemDetail(w, http.StatusMethodNotAllowed,
+			"about:blank#method-not-allowed", "Method Not Allowed",
+			"POST only", nil)
+		return
+	}
+
+	if h.config.Auth.TokenURL == "" || h.config.Auth.ClientID == "" {
+		writeProblemDetail(w, http.StatusServiceUnavailable,
+			"about:blank#not-configured", "Not Configured",
+			"token endpoint not configured", nil)
+		return
+	}
+
+	var req tokenRequest
+	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+		writeProblemDetail(w, http.StatusBadRequest,
+			"about:blank#bad-request", "Bad Request",
+			"invalid JSON body", nil)
+		return
+	}
+
+	if req.Username == "" || req.Password == "" {
+		writeProblemDetail(w, http.StatusBadRequest,
+			"about:blank#bad-request", "Bad Request",
+			"username and password are required", nil)
+		return
+	}
+
+	scope := req.Scope
+	if scope == "" {
+		scope = "openid roles permissions"
+	}
+
+	form := url.Values{
+		"grant_type":    {"password"},
+		"username":      {req.Username},
+		"password":      {req.Password},
+		"client_id":     {h.config.Auth.ClientID},
+		"client_secret": {h.config.Auth.ClientSecret},
+		"scope":         {scope},
+	}
+
+	h.forwardTokenResponse(w, form, "/auth/token")
+}
+
+// handleAuthRefresh exchanges a refresh token for a new JWT.
+func (h *Handler) handleAuthRefresh(w http.ResponseWriter, r *http.Request) {
+	if r.Method != "POST" {
+		writeProblemDetail(w, http.StatusMethodNotAllowed,
+			"about:blank#method-not-allowed", "Method Not Allowed",
+			"POST only", nil)
+		return
+	}
+
+	if h.config.Auth.TokenURL == "" || h.config.Auth.ClientID == "" {
+		writeProblemDetail(w, http.StatusServiceUnavailable,
+			"about:blank#not-configured", "Not Configured",
+			"token endpoint not configured", nil)
+		return
+	}
+
+	var req refreshRequest
+	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+		writeProblemDetail(w, http.StatusBadRequest,
+			"about:blank#bad-request", "Bad Request",
+			"invalid JSON body", nil)
+		return
+	}
+
+	if req.RefreshToken == "" {
+		writeProblemDetail(w, http.StatusBadRequest,
+			"about:blank#bad-request", "Bad Request",
+			"refresh_token is required", nil)
+		return
+	}
+
+	form := url.Values{
+		"grant_type":    {"refresh_token"},
+		"refresh_token": {req.RefreshToken},
+		"client_id":     {h.config.Auth.ClientID},
+		"client_secret": {h.config.Auth.ClientSecret},
+	}
+
+	if req.Scope != "" {
+		form.Set("scope", req.Scope)
+	}
+
+	h.forwardTokenResponse(w, form, "/auth/refresh")
+}
+
+// forwardTokenResponse posts form data to the token endpoint and
+// forwards the response verbatim to the client.
+func (h *Handler) forwardTokenResponse(w http.ResponseWriter, form url.Values, logPath string) {
+	resp, err := h.authHTTP.PostForm(h.config.Auth.TokenURL, form)
+	if err != nil {
+		writeProblemDetail(w, http.StatusBadGateway,
+			"about:blank#bad-gateway", "Bad Gateway",
+			"identity provider unreachable", nil)
+		logging.Errorf("auth upstream error", err, map[string]string{
+			"path": logPath,
+		})
+		return
+	}
+	defer resp.Body.Close()
+
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(resp.StatusCode)
+	io.Copy(w, resp.Body)
+}