Phase 3: on-device Cashu wallet with mbedTLS secp256k1 + SPIFFS persistence + PSRAM

- wallet.c/h: secp256k1 ECP primitives (hash_to_curve, scalar_mul, point_add)
- wallet_persist.c/h: SPIFFS persistence with threshold-based write protection
- Fee accounting for swap (input_fee_ppk from /v1/keysets)
- Keyset fetch via /v1/keysets (586 bytes vs 21KB for /v1/keys)
- Wallet API: GET /wallet, POST /wallet/swap, POST /wallet/send
- Payment proofs auto-stored to wallet + persisted on SPIFFS
- PSRAM enabled for large allocations (ESP32-S3 has 8MB)
- Wallet init deferred to dedicated task (avoids sys_evt stack overflow)
- Cashu proof ID buffer size fixed (66 hex chars, not 16)
- HTTP client: added fetch_headers() call for proper response handling
- persist_threshold_sats config parameter (default: 1 sat)

1 files changed, 1 insertions, 0 deletions

diff --git a/main/config.h b/main/config.h
index dd3fe05..2bcd400 100644
--- a/main/config.h
+++ b/main/config.h
@@ -34,6 +34,7 @@ typedef struct {
     char lnurl_url[256];
     int price_per_step;
     int step_size_ms;
+    uint64_t persist_threshold_sats;
 
     bool unique_derived;
 } tollgate_config_t;