Mirror terdut-server's new API key expires_at field

terdut-server's api_keys can now optionally expire (expires_in_days on
creation). The JSON shape change is additive and the TUI doesn't create
or list keys with an expiry yet, so this is a pure decode-compatibility
mirror, not a feature: APIKey gains ExpiresAt, nil for a key that never
expires or when talking to an older server that never sends the field.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

internal/api/types.go

@@ -281,6 +281,10 @@ type APIKey struct {
 	Key        string     `json:"key,omitempty"` // only present in create response
 	CreatedAt  time.Time  `json:"created_at"`
 	LastUsedAt *time.Time `json:"last_used_at"`
+	// ExpiresAt is nil for a key that never expires, including on a server
+	// old enough to not send the field at all — same decoded result either
+	// way. Not yet surfaced anywhere in the TUI.
+	ExpiresAt *time.Time `json:"expires_at,omitempty"`
 }
 
 // SimilarIncident is an earlier, resolved incident with the same signature