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`, so clients can retry. Decode errors split: an over-limit
body returns 413 (via errors.As on `*http.MaxBytesError`), malformed
JSON stays 400. A new MaxBodyBytes middleware (1 MiB default) caps
every route — rejecting an oversized Content-Length up front and
capping the read otherwise — so the health check and future routes are
bounded too. The raw attacker-controlled geo blob is no longer logged,
only its length; client_id and timestamp are length-bounded before
logging. A decodeJSON handler helper is added. Panic recovery is now a
local middleware routing the stack through slog as structured JSON.
Storage failure uses 500: a full buffer or write error is server-side
and retryable.

Model: opus-4-8

backend/internal/middleware/middleware.go

@@ -3,11 +3,14 @@
 package middleware
 
 import (
+	"errors"
 	"fmt"
+	"io"
 	"log/slog"
 	"net"
 	"net/http"
 	"net/netip"
+	"runtime/debug"
 	"strings"
 	"time"
 
@@ -22,6 +25,14 @@ import (
 
 const corsMaxAgeSec = 300
 
+// jsonErrorBody is the body written for errors raised inside
+// middleware, matching the {"status":"error"} shape the handlers
+// return so clients see one error contract across the API.
+const (
+	jsonContentType = "application/json; charset=utf-8"
+	jsonErrorBody   = "{\"status\":\"error\"}\n"
+)
+
 // Security header values. The backend is a JSON API with no
 // HTML surface, so the CSP forbids every resource type and
 // framing outright.
@@ -236,6 +247,79 @@ func (s *Middleware) SecurityHeaders() func(http.Handler) http.Handler {
 	}
 }
 
+// writeJSONError writes the shared JSON error body with the
+// given status. Used where middleware must reject a request
+// before it reaches a handler.
+func writeJSONError(w http.ResponseWriter, status int) {
+	w.Header().Set("Content-Type", jsonContentType)
+	w.WriteHeader(status)
+	_, _ = io.WriteString(w, jsonErrorBody)
+}
+
+// MaxBodyBytes returns middleware that caps the request body at
+// limit bytes. A declared Content-Length over the limit is
+// rejected immediately with 413. Bodies without a declared
+// length (or that understate it) are capped as they are read, so
+// a handler that reads the body sees a *http.MaxBytesError it can
+// map to 413. Mount it with a different limit on a route group
+// that needs a different bound.
+func (s *Middleware) MaxBodyBytes(
+	limit int64,
+) func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(
+			func(w http.ResponseWriter, r *http.Request) {
+				if r.ContentLength > limit {
+					writeJSONError(
+						w,
+						http.StatusRequestEntityTooLarge,
+					)
+
+					return
+				}
+
+				r.Body = http.MaxBytesReader(w, r.Body, limit)
+
+				next.ServeHTTP(w, r)
+			},
+		)
+	}
+}
+
+// Recoverer returns middleware that recovers from a panic in a
+// downstream handler, logs the panic and stack trace through
+// slog, and responds 500 with no body. http.ErrAbortHandler is
+// re-panicked so the server can abort the response as intended.
+func (s *Middleware) Recoverer() func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(
+			func(w http.ResponseWriter, r *http.Request) {
+				defer func() {
+					rec := recover()
+					if rec == nil {
+						return
+					}
+
+					err, ok := rec.(error)
+					if ok && errors.Is(err, http.ErrAbortHandler) {
+						panic(rec)
+					}
+
+					s.log.ErrorContext(r.Context(),
+						"panic recovered",
+						"panic", fmt.Sprintf("%v", rec),
+						"stack", string(debug.Stack()),
+					)
+
+					w.WriteHeader(http.StatusInternalServerError)
+				}()
+
+				next.ServeHTTP(w, r)
+			},
+		)
+	}
+}
+
 // CORS returns middleware that adds permissive CORS headers.
 func (s *Middleware) CORS() func(http.Handler) http.Handler {
 	return cors.Handler(cors.Options{