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, 4 insertions, 0 deletions

diff --git a/main/config.c b/main/config.c
index d7837bc..7e8a14c 100644
--- a/main/config.c
+++ b/main/config.c
@@ -19,6 +19,7 @@ esp_err_t tollgate_config_init(void)
     g_config.ap_max_conn = 4;
     g_config.price_per_step = 21;
     g_config.step_size_ms = 60000;
+    g_config.persist_threshold_sats = 1;
 
     esp_vfs_spiffs_conf_t conf = {
         .base_path = "/spiffs",
@@ -117,6 +118,9 @@ esp_err_t tollgate_config_init(void)
     cJSON *step = cJSON_GetObjectItem(root, "step_size_ms");
     if (step) g_config.step_size_ms = step->valueint;
 
+    cJSON *persist = cJSON_GetObjectItem(root, "persist_threshold_sats");
+    if (persist) g_config.persist_threshold_sats = (uint64_t)persist->valuedouble;
+
     cJSON_Delete(root);
     ESP_LOGI(TAG, "Config loaded: AP='%s', %d WiFi networks, price=%d sats/%dms",
              g_config.ap_ssid, g_config.network_count, g_config.price_per_step, g_config.step_size_ms);