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-iam/src/infrastructure/persistence/session_lock_repo.rs

@@ -98,7 +98,8 @@ impl SessionLockRepository for FileSystemSessionLockRepository {
         // SAFETY: `libc::kill(pid, 0)` sends no signal; it only probes
         // whether the process exists and is signalable by us.
         // PIDs on Linux fit in i32 (default pid_max ≈ 4 million).
-        let pid_signed: i32 = pid.try_into().unwrap_or(0);
+        let pid_signed: i32 = pid.try_into()
+            .expect("PID exceeds i32 range — kernel pid_max > 2^31");
         if unsafe { libc::kill(pid_signed, 0) != 0 } {
             return false;
         }