feat(api): add v2 routes, auth middleware, structured errors, and server status endpoint

Introduces incremental API improvements for custom launcher support
(mhf-iel, stratic-dev's Rust launcher):

- Standardize all error responses to JSON envelopes with error/message
- Add Bearer token auth middleware for v2 routes (legacy body-token preserved)
- Add `returning` (>90d inactive) and `courses` fields to auth response
- Add /v2/ route prefix with HTTP method enforcement
- Add GET /v2/server/status for MezFes, featured weapon, and event status
- Add APIEventRepo for read-only event data access

Closes #44

server/api/errors.go

@@ -0,0 +1,21 @@
+package api
+
+import (
+	"encoding/json"
+	"net/http"
+)
+
+// ErrorResponse is the standard JSON error envelope returned by all API endpoints.
+type ErrorResponse struct {
+	Error   string `json:"error"`
+	Message string `json:"message"`
+}
+
+func writeError(w http.ResponseWriter, status int, code, message string) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	_ = json.NewEncoder(w).Encode(ErrorResponse{
+		Error:   code,
+		Message: message,
+	})
+}