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, 18 insertions, 8 deletions

diff --git a/main/tollgate_client.c b/main/tollgate_client.c
index a81d16f..73c8370 100644
--- a/main/tollgate_client.c
+++ b/main/tollgate_client.c
@@ -127,15 +127,25 @@ static bool parse_discovery_response(const char *json_str, tollgate_discovery_t
             if (val && cJSON_IsString(val)) {
                 out->step_size_ms = atoi(val->valuestring);
             }
-        } else if (strcmp(tag_name->valuestring, "price_per_step") == 0 && tag_len >= 6) {
-            cJSON *amount = cJSON_GetArrayItem(tag, 2);
-            cJSON *mint = cJSON_GetArrayItem(tag, 4);
+        } else if (strcmp(tag_name->valuestring, "price_per_step") == 0 && tag_len >= 4) {
+            cJSON *payment_type = cJSON_GetArrayItem(tag, 2);
+
+            if (cJSON_IsString(payment_type) && strcmp(payment_type->valuestring, "mining") == 0 && tag_len >= 5) {
+                out->mining_available = true;
+                cJSON *port_val = cJSON_GetArrayItem(tag, 3);
+                if (port_val && cJSON_IsString(port_val)) {
+                    out->mining_port = (uint16_t)atoi(port_val->valuestring);
+                }
+            } else {
+                cJSON *amount = cJSON_GetArrayItem(tag, 2);
+                cJSON *mint = cJSON_GetArrayItem(tag, 4);
 
-            if (amount && cJSON_IsString(amount)) {
-                out->price_per_step = atoi(amount->valuestring);
-            }
-            if (mint && cJSON_IsString(mint)) {
-                strncpy(out->mint_url, mint->valuestring, sizeof(out->mint_url) - 1);
+                if (amount && cJSON_IsString(amount)) {
+                    out->price_per_step = atoi(amount->valuestring);
+                }
+                if (mint && cJSON_IsString(mint)) {
+                    strncpy(out->mint_url, mint->valuestring, sizeof(out->mint_url) - 1);
+                }
             }
         }
     }