websocket: remove Config::getInstance usage

websocket(config): simplify config loading via core::Config

- remove redundant has() checks
- rely directly on getInt/getBool with defaults
- align with .env-based configuration from core
- ensure consistent mapping (websocket.* -> WEBSOCKET_*)

Keeps websocket fully decoupled from env and delegates config resolution to core.

refactor(websocket): align with vix::http namespace

- migrate HttpApi and internal usage from vix::vhttp to vix::http
- update examples to reflect new HTTP layer
- adjust metrics integration
- update changelog

no behavior change

fix(websocket): stabilize session lifecycle by disabling unsafe heart…

…beat and idle timer threads

- disable heartbeat thread (maybe_start_heartbeat)
- disable idle timer (arm_idle_timer)
- prevent glibc pthread priority crash
- improve shutdown behavior (Ctrl+C responsiveness)
- ensure safe open/close lifecycle for sessions

This change prioritizes correctness and stability over background timers.
Timers will be reintroduced later using executor-based scheduling.

fix(websocket): normalize SQLite target to SQLite3::SQLite3 and remov…

…e deprecated usage

refactor(http): remove noisy internal debug logs from shutdown path