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 {"status":"ok"}, so clients can retry. Decode errors are split:
an over-limit body returns 413 (via errors.As on *http.MaxBytesError),
malformed JSON stays 400. A new MaxBodyBytes middleware caps every
route (1 MiB default; rejects an oversized Content-Length up-front and
caps the read otherwise), replacing the per-route reader so the health
check and future routes are bounded too. The raw attacker-controlled
geo blob is no longer logged — only its byte length — and client_id and
timestamp are length-bounded before logging. A decodeJSON handler helper
is added per the HTTP server conventions. Panic recovery is now a local
middleware that routes the stack through slog as structured JSON rather
than chi's plain-text stderr. Error bodies still leak nothing internal.

Chosen failure code for a storage failure: 500, since a full buffer or
write error is server-side and retryable, not the client's fault.

Model: opus-4-8

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)
+}