Fix payload cap: double base64 inflation broke files near the old 10MB limit

The old 10MB cap was checked against the wire body, but two base64 layers
sit between the raw file and that body: crypto.js base64-encodes the file
before AEAD encryption, then nym.Client.Send base64-encodes the ciphertext
again to embed it in the JSON sent to the local nym-client over its control
WebSocket. A file at exactly the old cap produced a wire body just over the
limit after the first layer, and would have blown nym-client's own hardcoded
16MB WebSocket message limit after the second, surfacing as an opaque 500.

Lower the wire-body cap to 11MB, which keeps the second base64 layer with
real margin under nym-client's 16MB ceiling. Effective safe raw file size
is now roughly 8MB, not 10MB.

Reported by Ch1ffr3punk on nym.forum, reproduced with an exact 10MB file.

2 files changed, 13 insertions, 2 deletions

diff --git a/internal/handler/submit.go b/internal/handler/submit.go
index 2822488..712d6a5 100644
--- a/internal/handler/submit.go
+++ b/internal/handler/submit.go
@@ -29,7 +29,7 @@ func (s *Submit) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	r.Body = http.MaxBytesReader(w, r.Body, 10*1024*1024)
+	r.Body = http.MaxBytesReader(w, r.Body, 11*1024*1024) // wire size, see relay.maxPayloadBytes
 	defer r.Body.Close()
 
 	if err := s.relay.Forward(r.Body); err != nil {
diff --git a/internal/relay/relay.go b/internal/relay/relay.go
index 9cd0366..9a17fab 100644
--- a/internal/relay/relay.go
+++ b/internal/relay/relay.go
@@ -8,7 +8,18 @@ import (
 	"nymdrop/internal/nym"
 )
 
-const maxPayloadBytes = 10 * 1024 * 1024 // 10 MB
+// Two base64 layers stack between the raw file and the wire: the client
+// base64-encodes the file into the plaintext before encrypting (see
+// static/crypto.js, ~4/3 inflation), then nym.Client.Send base64-encodes
+// the whole ciphertext again to embed it in the JSON message sent to the
+// local nym-client over its control WebSocket (~4/3 again). The combined
+// ~1.78x inflation is bounded above by nym-client's own hardcoded 16MB
+// WebSocket message limit, not by anything in this codebase, so this
+// buffer (and the nginx/http-layer cap) must stay well under 16MB / 1.78
+// once the second layer is applied. 11MB here keeps the second-layer
+// base64 (~14.7MB) with real margin below the 16MB nym-client ceiling,
+// which works out to a safe raw file size of roughly 8MB, not 10MB.
+const maxPayloadBytes = 11 * 1024 * 1024 // 11 MB wire (first base64 layer only)
 
 // Relay receives ciphertext and forwards it through Nym. Nothing is written to disk.
 type Relay struct {