feat: ConfigChanged event — runtime config propagation via EventBus (#622)

[ShaerWare]

Summary

• Add ConfigChanged event in modules/core/events.py — fields: key, value, previous_value, namespace
• ConfigService.set(), set_telegram(), set_widget() publish ConfigChanged after DB commit
• Audit subscriber in setup_event_subscriptions() logs all config changes (action=config_changed)
• Namespace derived from key (e.g. widget.colors.primary → widget) for subscriber filtering
• Graceful fallback: event publishing skipped if EventBus unavailable (startup, tests)
• previous_value fetched from DB before write (not from Redis cache) for accuracy

Design decisions

• Audit via EventBus replaces manual audit at call sites (widget/telegram routers still have their own audit for now — can be removed later)
• Domain subscribers not added yet — widget reads config per-request (no cache to invalidate), telegram bot is a subprocess (can't hot-reload). Future domain events (BotConfigUpdated, WidgetConfigUpdated) are better suited for instance-specific changes
• Idempotent handlers — repeated events with same value produce independent audit entries, no side effects

Test plan

• 9 new unit tests pass (pytest tests/unit/test_config_changed.py)
• All 106 unit tests pass (no regressions)
• CI checks (lint-backend, lint-frontend, security)

Closes #622

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