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, 103 insertions, 0 deletions

diff --git a/tests/unit/test_tollgate_client_mining.c b/tests/unit/test_tollgate_client_mining.c
new file mode 100644
index 0000000..e270864
--- /dev/null
+++ b/tests/unit/test_tollgate_client_mining.c
@@ -0,0 +1,103 @@
+#include "test_framework.h"
+#include "../../main/config.h"
+#include <string.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <cjson/cJSON.h>
+
+static tollgate_config_t g_test_config;
+
+const tollgate_config_t *tollgate_config_get(void) {
+    return &g_test_config;
+}
+
+uint64_t nucula_wallet_balance(void) { return 100; }
+esp_err_t nucula_wallet_send(uint64_t a, char *b, size_t c) { (void)a; (void)b; (void)c; return ESP_OK; }
+
+#include "freertos/FreeRTOS.h"
+
+#include "../../main/tollgate_client.c"
+
+int main(void)
+{
+    printf("=== test_tollgate_client_mining ===\n");
+
+    memset(&g_test_config, 0, sizeof(g_test_config));
+    g_test_config.client_enabled = true;
+
+    printf("\n--- mining tag: mining_available=true, port=3333 ---\n");
+    {
+        const char *json = "{\"kind\":10021,\"tags\":["
+            "[\"metric\",\"milliseconds\"],"
+            "[\"step_size\",\"60000\"],"
+            "[\"price_per_step\",\"0\",\"mining\",\"3333\",\"sat\"],"
+            "[\"tips\",\"1\",\"2\",\"5\"]"
+            "]}";
+
+        tollgate_discovery_t disc;
+        bool ok = parse_discovery_response(json, &disc);
+        ASSERT(ok, "mining discovery parsed");
+        ASSERT(disc.is_tollgate, "is_tollgate=true");
+        ASSERT(disc.mining_available, "mining_available=true");
+        ASSERT_EQ_INT(3333, (int)disc.mining_port, "mining_port=3333");
+    }
+
+    printf("\n--- mining tag: no mining tag ---\n");
+    {
+        const char *json = "{\"kind\":10021,\"tags\":["
+            "[\"metric\",\"milliseconds\"],"
+            "[\"step_size\",\"60000\"],"
+            "[\"price_per_step\",\"cashu\",\"21\",\"sat\",\"https://testnut.cashu.space\",\"1\"]"
+            "]}";
+
+        tollgate_discovery_t disc;
+        bool ok = parse_discovery_response(json, &disc);
+        ASSERT(ok, "cashu discovery parsed");
+        ASSERT(disc.is_tollgate, "is_tollgate=true");
+        ASSERT(!disc.mining_available, "mining_available=false");
+        ASSERT_EQ_INT(0, (int)disc.mining_port, "mining_port=0 when no mining");
+        ASSERT_EQ_INT(21, disc.price_per_step, "price_per_step=21 for cashu");
+    }
+
+    printf("\n--- mining tag: custom port 4033 ---\n");
+    {
+        const char *json = "{\"kind\":10021,\"tags\":["
+            "[\"metric\",\"milliseconds\"],"
+            "[\"step_size\",\"60000\"],"
+            "[\"price_per_step\",\"0\",\"mining\",\"4033\",\"sat\"]"
+            "]}";
+
+        tollgate_discovery_t disc;
+        bool ok = parse_discovery_response(json, &disc);
+        ASSERT(ok, "mining custom port parsed");
+        ASSERT(disc.mining_available, "mining_available=true");
+        ASSERT_EQ_INT(4033, (int)disc.mining_port, "mining_port=4033");
+    }
+
+    printf("\n--- tollgate_discovery_t zero-init ---\n");
+    {
+        tollgate_discovery_t disc = {0};
+        ASSERT(!disc.is_tollgate, "zero-init: is_tollgate=false");
+        ASSERT(!disc.mining_available, "zero-init: mining_available=false");
+        ASSERT_EQ_INT(0, (int)disc.mining_port, "zero-init: mining_port=0");
+        ASSERT_EQ_INT(0, disc.price_per_step, "zero-init: price=0");
+    }
+
+    printf("\n--- TG_CLIENT_MINING state enum ---\n");
+    {
+        ASSERT(TG_CLIENT_MINING > TG_CLIENT_PAID, "MINING > PAID in enum");
+        ASSERT(TG_CLIENT_MINING < TG_CLIENT_ERROR, "MINING < ERROR in enum");
+    }
+
+    printf("\n--- discovery struct fields ---\n");
+    {
+        tollgate_discovery_t disc;
+        memset(&disc, 0, sizeof(disc));
+        disc.mining_available = true;
+        disc.mining_port = 9999;
+        ASSERT(disc.mining_available, "mining_available set");
+        ASSERT_EQ_INT(9999, (int)disc.mining_port, "mining_port set");
+    }
+
+    TEST_SUMMARY();
+}