fix(backend): report ingest correctness — propagate storage failure, 413 on oversize, global body cap (closes #23)

A buffer failure on POST /api/v1/reports now returns 500 instead of a
false `ok`: the failure is server-side and a client can retry. An
over-limit body returns 413 (errors.As on `*http.MaxBytesError`);
malformed JSON stays 400. A MaxBodyBytes middleware (1 MiB) caps every
route; a route group can only lower that limit. The raw geo blob is no
longer logged, only its length; client_id, timestamp and decode error
text are length-bounded before logging. A decodeJSON handler helper is
added. Panic recovery routes the stack through slog as structured
JSON. Writing a report file now returns its error, so a failed final
flush fails the stop and the process exits non-zero.

Model: opus-5-5

backend/internal/handlers/handlers.go

@@ -18,6 +18,13 @@ import (
 
 const jsonContentType = "application/json; charset=utf-8"
 
+// reportAppender is the subset of the report buffer the handlers
+// depend on. Defining it here keeps the storage failure path
+// exercisable with a stub in tests.
+type reportAppender interface {
+	Append(v any) error
+}
+
 // Params defines the dependencies for Handlers.
 type Params struct {
 	fx.In
@@ -30,7 +37,7 @@ type Params struct {
 
 // Handlers provides HTTP handler factories for all endpoints.
 type Handlers struct {
-	buf    *reportbuf.Buffer
+	buf    reportAppender
 	hc     *healthcheck.Healthcheck
 	log    *slog.Logger
 	params *Params
@@ -72,3 +79,15 @@ func (s *Handlers) respondJSON(
 		}
 	}
 }
+
+// decodeJSON decodes the request body into v. The body is
+// expected to already be bounded by the body-size middleware, so
+// a caller can distinguish an over-limit body from malformed
+// JSON by testing the returned error for *http.MaxBytesError.
+func (s *Handlers) decodeJSON(
+	_ http.ResponseWriter,
+	r *http.Request,
+	v any,
+) error {
+	return json.NewDecoder(r.Body).Decode(v)
+}