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, 47 insertions, 0 deletions

diff --git a/main/firewall.c b/main/firewall.c
index 8d535b4..ae0eda7 100644
--- a/main/firewall.c
+++ b/main/firewall.c
@@ -7,12 +7,16 @@
 #include "lwip/etharp.h"
 #include "lwip/netif.h"
 #include "lwip/prot/ip4.h"
+#include "lwip/prot/tcp.h"
+#include "lwip/prot/ip.h"
 #include <string.h>
 
 #define MAX_CLIENTS 10
 
 static const char *TAG = "firewall";
 static esp_ip4_addr_t s_ap_ip;
+static uint16_t s_mining_port = 3333;
+static bool s_sandbox_mint_access = false;
 
 typedef struct {
     uint32_t ip;
@@ -66,6 +70,46 @@ esp_err_t firewall_init(esp_ip4_addr_t ap_ip)
     return ESP_OK;
 }
 
+void firewall_set_mining_port(uint16_t port)
+{
+    s_mining_port = port;
+}
+
+void firewall_set_sandbox_mint_access(bool enabled)
+{
+    s_sandbox_mint_access = enabled;
+}
+
+static bool is_sandbox_allowed(struct pbuf *p)
+{
+    if (p->len < IP_HLEN) return false;
+    struct ip_hdr *iphdr = (struct ip_hdr *)p->payload;
+    uint32_t dest_ip_h = lwip_ntohl(iphdr->dest.addr);
+    uint32_t ap_ip_h = lwip_ntohl(s_ap_ip.addr);
+
+    if (dest_ip_h == ap_ip_h) {
+        if (iphdr->_proto == IP_PROTO_TCP) {
+            uint16_t dst_port = 0;
+            if (p->len >= IP_HLEN + TCP_HLEN) {
+                struct tcp_hdr *tcphdr = (struct tcp_hdr *)((uint8_t *)p->payload + IP_HLEN);
+                dst_port = lwip_ntohs(tcphdr->dest);
+            }
+            if (dst_port == 80 || dst_port == 2121 || dst_port == s_mining_port) {
+                return true;
+            }
+        }
+        if (iphdr->_proto == IP_PROTO_UDP) {
+            return true;
+        }
+    }
+
+    if (s_sandbox_mint_access && iphdr->_proto == IP_PROTO_TCP) {
+        return true;
+    }
+
+    return false;
+}
+
 int tollgate_ip4_canforward_filter(struct pbuf *p, u32_t dest_addr_hostorder)
 {
     (void)dest_addr_hostorder;
@@ -79,6 +123,9 @@ int tollgate_ip4_canforward_filter(struct pbuf *p, u32_t dest_addr_hostorder)
     if (firewall_is_client_allowed(iphdr->src.addr)) {
         return 1;
     }
+    if (is_sandbox_allowed(p)) {
+        return 1;
+    }
     return 0;
 }