refactor: use AuthStore interface and remove type assertions

Replace runtime type assertions with compile-time safe AuthStore interface.
Add connState struct for cleaner per-connection state management instead
of mutable pointer parameters. Reduce auth challenge TTL from 10min to 2min.

- Add AuthStore interface with CreateAuthChallenge and ValidateAndConsumeChallenge
- Add connState struct for authenticatedPubkey and authChallenge
- Remove fragile type assertion pattern in requireAuth and handleAuth
- Add nil checks for auth store with clear error messages
- Update Handler to have separate auth field
- Wire auth store in main.go when auth is enabled

1 files changed, 1 insertions, 1 deletions

diff --git a/internal/storage/auth.go b/internal/storage/auth.go
index 6eefa41..e17ffeb 100644
--- a/internal/storage/auth.go
+++ b/internal/storage/auth.go
@@ -10,7 +10,7 @@ import (
 
 const (
         ChallengeLength = 32 // bytes
-        ChallengeTTL    = 10 * time.Minute
+        ChallengeTTL    = 2 * time.Minute
 )
 
 func generateChallenge() (string, error) {