feat(auth): extend refresh TTL and keep session on transient errors

Bumps refresh token lifetime to 720h with env overrides, and stops
clearing the local session for non-401 refresh failures so users aren't
logged out by transient network blips. Adds a 60s retry timer when a
refresh fails for non-auth reasons.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/internal/shared/config/config.go

@@ -376,6 +376,15 @@ func applyEnvOverrides(cfg *Config) {
 		return
 	}
 
+	if secret, ok := lookupNonEmptyEnv("JWT_SECRET"); ok {
+		cfg.JWT.Secret = secret
+	}
+	if ttl, ok := lookupDurationEnv("JWT_ACCESS_TTL"); ok {
+		cfg.JWT.AccessTTL = ttl
+	}
+	if ttl, ok := lookupDurationEnv("JWT_REFRESH_TTL"); ok {
+		cfg.JWT.RefreshTTL = ttl
+	}
 	if apiKey, ok := lookupNonEmptyEnv("LLM_API_KEY"); ok {
 		cfg.LLM.APIKey = apiKey
 	}
@@ -784,6 +793,19 @@ func lookupBoolEnv(key string) (bool, bool) {
 	}
 }
 
+func lookupDurationEnv(key string) (time.Duration, bool) {
+	value, ok := lookupNonEmptyEnv(key)
+	if !ok {
+		return 0, false
+	}
+
+	duration, err := time.ParseDuration(value)
+	if err != nil {
+		return 0, false
+	}
+	return duration, true
+}
+
 func maxInt(value, fallback int) int {
 	if value < fallback {
 		return fallback