Phase 3: Nostr identity derivation + wifistr service discovery

- Add identity.c/h: HMAC-SHA512 derivation from nsec → npub, STA/AP MAC, SSID, AP IP
- Add nostr_event.c/h: NIP-01 event serialization + Schnorr signing (BIP-340)
- Add geohash.c/h: lat/lon to geohash encoding
- Add wifistr.c/h: kind 38787 event builder + WebSocket publish to Nostr relays
- Update config.c/h: nsec-based identity, Nostr relay/geo config, remove static SSID/IP
- Replace custom mbedTLS wallet with nucula library (libsecp256k1)
- Remove wallet.c/h, wallet_persist.c/h (replaced by nucula_lib component)
- Verified on Board A: derived SSID, captive portal, payment, wallet, wifistr publish

1 files changed, 25 insertions, 0 deletions

diff --git a/main/nostr_event.h b/main/nostr_event.h
new file mode 100644
index 0000000..ce15900
--- /dev/null
+++ b/main/nostr_event.h
@@ -0,0 +1,25 @@
+#ifndef NOSTR_EVENT_H
+#define NOSTR_EVENT_H
+
+#include "esp_err.h"
+#include <stdint.h>
+#include <stddef.h>
+
+typedef struct {
+    char pubkey[65];
+    uint64_t created_at;
+    int kind;
+    const char *tags_json;
+    const char *content;
+    char id[65];
+    char sig[129];
+} nostr_event_t;
+
+esp_err_t nostr_event_init(nostr_event_t *event, const char *npub_hex,
+                           int kind, const char *tags_json, const char *content);
+
+esp_err_t nostr_event_sign(nostr_event_t *event, const uint8_t nsec[32]);
+
+esp_err_t nostr_event_to_json(const nostr_event_t *event, char *buf, size_t buf_len);
+
+#endif