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, 92 insertions, 0 deletions

diff --git a/tests/unit/test_mining_payment.c b/tests/unit/test_mining_payment.c
new file mode 100644
index 0000000..c3834fd
--- /dev/null
+++ b/tests/unit/test_mining_payment.c
@@ -0,0 +1,92 @@
+#include "test_framework.h"
+#include "../../main/mining_payment.h"
+#include <stdio.h>
+#include <string.h>
+#include <math.h>
+
+int main(void)
+{
+    printf("=== test_mining_payment ===\n");
+
+    mining_payment_init();
+
+    printf("\n--- mining_nbits_to_difficulty ---\n");
+    uint64_t d1 = mining_nbits_to_difficulty(0);
+    ASSERT(d1 == UINT64_MAX, "nbits=0 returns max");
+
+    uint64_t d2 = mining_nbits_to_difficulty(0x170309E2);
+    ASSERT(d2 > 0, "mainnet nbits gives positive difficulty");
+    printf("  INFO: difficulty for 0x170309E2 = %llu\n", (unsigned long long)d2);
+
+    uint64_t d3 = mining_nbits_to_difficulty(0x1d00ffff);
+    ASSERT(d3 == 1, "pseudotransaction nbits = difficulty 1");
+
+    printf("\n--- mining_calculate_hashprice ---\n");
+    double hp1 = mining_calculate_hashprice(0);
+    ASSERT(hp1 == 0.0, "nbits=0 gives hashprice 0");
+
+    double hp2 = mining_calculate_hashprice(0x170309E2);
+    ASSERT(hp2 > 0.0, "mainnet nbits gives positive hashprice");
+    printf("  INFO: hashprice for 0x170309E2 = %.6f sat/GH/s/day\n", hp2);
+
+    printf("\n--- mining_calculate_hashprice_override ---\n");
+    double hp3 = mining_calculate_hashprice_override(1000);
+    ASSERT(fabs(hp3 - 1000.0) < 0.001, "override returns exact value");
+
+    printf("\n--- mining_set_current_nbits ---\n");
+    mining_set_current_nbits(0x170309E2);
+    double hp4 = mining_get_current_hashprice();
+    ASSERT(fabs(hp4 - hp2) < 0.0001, "stored hashprice matches calculated");
+
+    printf("\n--- mining_get_or_create_client ---\n");
+    mining_client_stats_t *c1 = mining_get_or_create_client(0x0A010203);
+    ASSERT(c1 != NULL, "created client 1");
+    ASSERT(c1->ip == 0x0A010203, "client 1 IP matches");
+    ASSERT(c1->shares_accepted == 0, "client 1 starts with 0 shares");
+
+    mining_client_stats_t *c2 = mining_get_or_create_client(0x0A010204);
+    ASSERT(c2 != NULL, "created client 2");
+    ASSERT(c2->ip == 0x0A010204, "client 2 IP matches");
+
+    mining_client_stats_t *c1_again = mining_get_or_create_client(0x0A010203);
+    ASSERT(c1_again == c1, "same IP returns same client");
+
+    printf("\n--- mining_update_hashrate ---\n");
+    mining_update_hashrate(0x0A010203, true);
+    mining_update_hashrate(0x0A010203, true);
+    mining_update_hashrate(0x0A010203, false);
+
+    const mining_client_stats_t *c1_stats = mining_get_client_stats(0x0A010203);
+    ASSERT(c1_stats != NULL, "client 1 stats found");
+    ASSERT(c1_stats->shares_accepted == 2, "client 1 has 2 accepted");
+    ASSERT(c1_stats->shares_rejected == 1, "client 1 has 1 rejected");
+
+    printf("\n--- mining_get_client_stats ---\n");
+    const mining_client_stats_t *notfound = mining_get_client_stats(0xFFFFFFFF);
+    ASSERT(notfound == NULL, "nonexistent client returns NULL");
+
+    printf("\n--- mining_shares_to_allotment_ms ---\n");
+    uint64_t allot1 = mining_shares_to_allotment_ms(0.0, 100.0, 21, 60000);
+    ASSERT(allot1 == 0, "zero hashrate = zero allotment");
+
+    uint64_t allot2 = mining_shares_to_allotment_ms(1.0, 100.0, 21, 60000);
+    ASSERT(allot2 > 0, "positive hashrate gives positive allotment");
+    printf("  INFO: 1 GH/s at 100 sat/GH/s/day, 21 sats/60s => %llu ms\n", (unsigned long long)allot2);
+
+    uint64_t allot3 = mining_shares_to_allotment_ms(10.0, 50.0, 10, 30000);
+    ASSERT(allot3 > 0, "10 GH/s at 50 sat gives positive allotment");
+    printf("  INFO: 10 GH/s at 50 sat/GH/s/day, 10 sats/30s => %llu ms\n", (unsigned long long)allot3);
+
+    printf("\n--- mining_shares_to_allotment_bytes ---\n");
+    uint64_t ab1 = mining_shares_to_allotment_bytes(0.0, 100.0, 21, 1048576);
+    ASSERT(ab1 == 0, "zero hashrate = zero bytes");
+
+    uint64_t ab2 = mining_shares_to_allotment_bytes(1.0, 100.0, 21, 1048576);
+    ASSERT(ab2 > 0, "positive hashrate gives positive bytes");
+
+    printf("\n--- mining_validate_share (stub) ---\n");
+    esp_err_t vr = mining_validate_share(NULL, 0, NULL, 0);
+    ASSERT(vr == ESP_OK, "validate share stub returns OK");
+
+    TEST_SUMMARY();
+}