add default keys w/public

allows default keys and future extra keys from config

maybe add some other live decoding or something too, store
hashtag channel keys in the db?

internal/keystore/keystore.go

@@ -3,13 +3,23 @@
 // Future: add a DB-backed fallback that checks the channel_keys table.
 package keystore
 
-import "encoding/hex"
+import (
+	"encoding/hex"
+	"maps"
+)
 
 type MapKeyStore struct {
 	keys map[string][]byte // channel hash hex string → key bytes
 }
 
-func NewMapKeyStore(keys map[string][]byte) *MapKeyStore {
+var defaultKeys = map[string][]byte{
+	"11": {0x8b, 0x33, 0x87, 0xe9, 0xc5, 0xcd, 0xea, 0x6a, 0xc9, 0xe5, 0xed, 0xba, 0xa1, 0x15, 0xcd, 0x72}, // public channel
+}
+
+func NewMapKeyStore(extra map[string][]byte) *MapKeyStore {
+	keys := make(map[string][]byte)
+	maps.Copy(keys, defaultKeys)
+	maps.Copy(keys, extra)
 	return &MapKeyStore{keys}
 }