feat(phase6): bytes-based billing - dual metric support

- session_create_bytes() + session_add_bytes() for bytes-metric sessions
- session_is_expired() dispatches on config metric (bytes vs milliseconds)
- cashu_calculate_allotment() unified dispatcher for both metrics
- tollgate_api discovery/usage/session_event use configured metric
- config: metric field defaults to 'bytes', step_size_bytes=22020096 (21MB)
- 14 new unit tests (148 total passing)
- ASSERT_EQ_UINT64 macro added to test framework

1 files changed, 2 insertions, 0 deletions

diff --git a/main/config.h b/main/config.h
index de9f856..86b5e1a 100644
--- a/main/config.h
+++ b/main/config.h
@@ -43,6 +43,8 @@ typedef struct {
     char lnurl_url[256];
     int price_per_step;
     int step_size_ms;
+    int step_size_bytes;
+    char metric[16];
     uint64_t persist_threshold_sats;
 
     char nostr_geohash[16];