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, 39 insertions, 0 deletions

diff --git a/main/stratum_proxy.h b/main/stratum_proxy.h
new file mode 100644
index 0000000..b940640
--- /dev/null
+++ b/main/stratum_proxy.h
@@ -0,0 +1,39 @@
+#ifndef STRATUM_PROXY_H
+#define STRATUM_PROXY_H
+
+#include "esp_err.h"
+#include <stdint.h>
+#include <stdbool.h>
+
+#define STRATUM_MAX_JOB_ID_LEN 32
+#define STRATUM_MAX_JOBS 4
+
+typedef struct {
+    uint32_t job_id;
+    uint8_t prevhash[32];
+    uint8_t merkle_root[32];
+    uint32_t ntime;
+    uint32_t nbits;
+    uint32_t version;
+    uint8_t target[32];
+    int target_len;
+    bool valid;
+} stratum_job_t;
+
+typedef struct {
+    double hashrate_ghs;
+    uint32_t nbits;
+    uint64_t total_shares;
+    uint64_t total_accepted;
+    uint64_t total_rejected;
+    double current_hashprice;
+    int active_miners;
+} stratum_proxy_stats_t;
+
+esp_err_t stratum_proxy_init(uint16_t port);
+void stratum_proxy_set_job(const stratum_job_t *job);
+const stratum_job_t *stratum_proxy_get_current_job(void);
+void stratum_proxy_get_stats(stratum_proxy_stats_t *stats);
+void stratum_proxy_stop(void);
+
+#endif