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, 16 insertions, 1 deletions

diff --git a/tests/unit/Makefile b/tests/unit/Makefile
index a06807f..edff73c 100644
--- a/tests/unit/Makefile
+++ b/tests/unit/Makefile
@@ -22,7 +22,7 @@ LDFLAGS := -lmbedcrypto -lcjson -lm
 
 SECP256K1_OBJ := secp256k1.o precomputed_ecmult.o precomputed_ecmult_gen.o
 
-TESTS := test_geohash test_identity test_nostr_event test_cashu test_session test_tollgate_client test_lnurl_pay test_lightning_payout test_mcp_handler test_nip04 test_cvm_server test_display test_negentropy_adapter test_beacon_price test_market test_mint_health
+TESTS := test_geohash test_identity test_nostr_event test_cashu test_session test_tollgate_client test_lnurl_pay test_lightning_payout test_mcp_handler test_nip04 test_cvm_server test_display test_negentropy_adapter test_beacon_price test_market test_mint_health test_mining_payment test_stratum_proxy test_session_payment_method test_tollgate_client_mining test_firewall_sandbox
 
 .PHONY: all test clean $(TESTS)
 
@@ -90,5 +90,20 @@ test_beacon_price: test_beacon_price.c $(REPO_ROOT)/main/beacon_price.c
 test_market: test_market.c $(REPO_ROOT)/main/market.c $(REPO_ROOT)/main/beacon_price.c
         $(CC) $(CFLAGS) -I $(REPO_ROOT)/main $< $(REPO_ROOT)/main/market.c $(REPO_ROOT)/main/beacon_price.c -o $@ $(LDFLAGS)
 
+test_mining_payment: test_mining_payment.c $(REPO_ROOT)/main/mining_payment.c
+        $(CC) $(CFLAGS) $< $(REPO_ROOT)/main/mining_payment.c -o $@ $(LDFLAGS)
+
+test_stratum_proxy: test_stratum_proxy.c $(REPO_ROOT)/main/stratum_proxy.c $(REPO_ROOT)/main/mining_payment.c
+        $(CC) $(CFLAGS) $< $(REPO_ROOT)/main/stratum_proxy.c $(REPO_ROOT)/main/mining_payment.c -o $@ $(LDFLAGS)
+
+test_session_payment_method: test_session_payment_method.c $(REPO_ROOT)/main/session.c $(REPO_ROOT)/main/cashu.c
+        $(CC) $(CFLAGS) $< $(REPO_ROOT)/main/session.c $(REPO_ROOT)/main/cashu.c -o $@ $(LDFLAGS)
+
+test_tollgate_client_mining: test_tollgate_client_mining.c
+        $(CC) $(CFLAGS) $< -o $@ $(LDFLAGS)
+
+test_firewall_sandbox: test_firewall_sandbox.c $(REPO_ROOT)/main/firewall.c
+        $(CC) $(CFLAGS) -include stubs/dns_server.h $< $(REPO_ROOT)/main/firewall.c -o $@ $(LDFLAGS)
+
 clean:
         rm -f $(TESTS) $(SECP256K1_OBJ)