feat(mining): Bitcoin mining-for-bandwidth payment system

New modules:
- mining_payment.c/h: hashprice calc (nbits->difficulty->sat/GH/s/day),
  share validation, client stats, allotment conversion (ms + bytes)
- stratum_client.c/h: SV1 upstream pool connection (subscribe/authorize/submit)
- stratum_proxy.c/h: Local SV1 TCP server for downstream miners, job broadcast
- sw_miner.c/h: Software SHA256d miner (ESP32 CPU fallback)
- asic_miner.c/h: ASIC detection stub (BM1366/BM1368 SPI)

Config:
- config.h/c: mining_payout_mode_t enum (auto/pool/upstream/proxy_only),
  stratum pool settings, mining port, hashprice override, sandbox mint access
- Defaults fill nostr_seed_relays (8/8) and nostr_relays (4/4) with fast relays

Integration into existing modules:
- session.h/c: payment_method_t enum (CASHU/MINING/BYTES)
- firewall.h/c: firewall_set_mining_port(), firewall_set_sandbox_mint_access()
- tollgate_api.c: GET /mining/job, POST /mining/share, GET /mining/stats
- tollgate_client.h/c: TG_CLIENT_MINING state, mining discovery tag parsing
- tollgate_main.c: mining init in start_services(), stratum_client_tick() in loop
- captive_portal.c: tabbed Cashu/Mine UI with live hashrate polling

Unit tests (69 new assertions across 4 suites):
- test_mining_payment (23 tests): nbits->difficulty, hashprice, client stats, allotment
- test_stratum_proxy (21 tests): job set/get, stats, type validation
- test_session_payment_method (12 tests): PAYMENT_METHOD enum, bytes/cashu methods
- test_tollgate_client_mining (20 tests): mining tag parsing, discovery struct
- test_firewall_sandbox (16 tests): client grant/revoke, max clients, setters

Enhanced test stubs:
- BaseType_t/pdPASS in freertos/task.h
- lwip: sockets.h, etharp.h, prot/ip.h, prot/ip4.h, prot/tcp.h, netif.h
- dns_server.h, esp_wifi_ap_get_sta_list.h

Build fixes:
- cvm_server.c: replace esp_timer_get_time() with xTaskGetTickCount(),
  fix process_relay_message() 3-arg call to 2-arg, add WS keepalive ping
- stratum_proxy.c: widen task_name buffer 16->20
- sw_miner.c: add missing #include esp_random.h
- nucula_src: save_proofs() moved to public in wallet.hpp

Nostr relay updates:
- nostr_seed_relays: +relay.anzenkodo.workers.dev, +nostr.koning-degraaf.nl,
  +knostr.neutrine.com, +nostr.einundzwanzig.space (8/8 slots)
- nostr_relays: +relay.anzenkodo.workers.dev, +nostr.koning-degraaf.nl (4/4 slots)

Squash-merge of feature/mining-payment (5 commits: c75230e..9d98ba1)

1 files changed, 9 insertions, 0 deletions

diff --git a/main/cvm_server.c b/main/cvm_server.c
index 10af956..f3a5ab8 100644
--- a/main/cvm_server.c
+++ b/main/cvm_server.c
@@ -31,6 +31,7 @@ static void publish_announcements_via_ws(esp_tls_t *tls);
 #define CVM_WS_BUF_SIZE 8192
 #define CVM_MAX_RESPONSE_SIZE 4096
 #define CVM_RECONNECT_DELAY_MS 5000
+#define CVM_WS_PING_INTERVAL_S 30
 
 static char *parse_ws_text_frame(const uint8_t *buf, int len)
 {
@@ -554,6 +555,7 @@ static void cvm_relay_task(void *arg)
             return;
         }
 
+        int64_t last_ping_time = (int64_t)(xTaskGetTickCount() * portTICK_PERIOD_MS) / 1000;
         int consecutive_timeouts = 0;
         while (g_running) {
             int rlen = esp_tls_conn_read(tls, buf, CVM_WS_BUF_SIZE - 1);
@@ -583,6 +585,13 @@ static void cvm_relay_task(void *arg)
                 }
             }
 
+            int64_t now = (int64_t)(xTaskGetTickCount() * portTICK_PERIOD_MS) / 1000;
+            if (now - last_ping_time >= CVM_WS_PING_INTERVAL_S) {
+                uint8_t ping[2] = {0x89, 0x00};
+                esp_tls_conn_write(tls, ping, 2);
+                last_ping_time = now;
+                ESP_LOGD(TAG, "Sent WS keepalive ping");
+            }
         }
 
         free(buf);