Refactor session ID handling and improve error management

- Introduced `SessionId` newtype for validated session identifiers, ensuring safety against path traversal attacks.
- Updated session repository methods to accept `SessionId` instead of raw strings, enhancing type safety.
- Removed redundant error handling in repository methods by leveraging the new `Error` type from `zesdex_utils`.
- Simplified atomic JSON write operations by eliminating unnecessary error conversions.
- Enhanced integer casting with a new `CastOr` trait for safer narrowing conversions.
- Removed deprecated error handling code and consolidated error types across the codebase.
- Updated HTTP handlers to utilize the new session ID validation, improving overall robustness.

crates/zesdex-backend/src/app/runtime/event_loop/mod.rs

@@ -6,8 +6,8 @@
 //! `FAST_POLL_MS` (8 ms) or `SLOW_POLL_MS` (100 ms). `is_idle()` reports
 //! whether the deadline has expired.
 use std::collections::VecDeque;
-use std::convert::TryInto;
 use std::time::{Duration, Instant};
+use zesdex_utils::CastOr;
 
 use crate::app::state::runtime::TurnEvent;
 use tracing;
@@ -62,7 +62,7 @@ impl EventLoop {
 
     /// Return `true` if the app has been idle for more than `IDLE_THRESHOLD_MS`.
     pub fn is_idle(&self) -> bool {
-        let elapsed: u64 = self.last_activity.elapsed().as_millis().try_into().unwrap_or(u64::MAX);
+        let elapsed: u64 = self.last_activity.elapsed().as_millis().cast_or(u64::MAX);
         elapsed > IDLE_THRESHOLD_MS
     }