refactor: remove GATEWAY_USER_TOKENS, fix review feedback

GATEWAY_USER_TOKENS never went to production — replaced entirely by
DB-backed user management via /api/admin/users and /api/tokens.

Removed:
- UserTokenConfig struct and GATEWAY_USER_TOKENS env var parsing
- user_tokens field from GatewayConfig
- GatewayChannel::new_multi_auth() constructor
- Env-var user migration block in main.rs (~90 lines)
- multi_tenant auto-detection from GATEWAY_USER_TOKENS (now runtime
  via db.has_any_users() in app.rs)

Review fixes (zmanian):
- User ID generation: UUID instead of display-name derivation (#1)
- Invitation accept moved to public router (no auth needed) (#3)
- libSQL get_invitation_by_hash aligned with postgres: filters
  status='pending' AND expires_at > now (#4)
- UUID parse: returns DatabaseError::Serialization instead of
  unwrap_or_default (#7)
- PostgreSQL SELECT * replaced with explicit column lists (#8)
- Sort order aligned (both backends use DESC) (#6)

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

src/config/heartbeat.rs

@@ -21,8 +21,8 @@ pub struct HeartbeatConfig {
     pub quiet_hours_end: Option<u32>,
     /// Timezone for fire_at and quiet hours evaluation (IANA name).
     pub timezone: Option<String>,
-    /// When true, cycle through all users with routines. Auto-detected from
-    /// GATEWAY_USER_TOKENS or set explicitly via HEARTBEAT_MULTI_TENANT.
+    /// When true, cycle through all users with routines. Set explicitly via
+    /// HEARTBEAT_MULTI_TENANT or detected at runtime after DB initialization.
     pub multi_tenant: bool,
 }
 
@@ -105,12 +105,7 @@ impl HeartbeatConfig {
                 }
                 tz
             },
-            // Auto-detect multi-tenant mode from GATEWAY_USER_TOKENS presence,
-            // or allow explicit override via HEARTBEAT_MULTI_TENANT.
-            multi_tenant: parse_bool_env(
-                "HEARTBEAT_MULTI_TENANT",
-                optional_env("GATEWAY_USER_TOKENS")?.is_some(),
-            )?,
+            multi_tenant: parse_bool_env("HEARTBEAT_MULTI_TENANT", false)?,
         })
     }
 }