chore: remove Shoutrrr residue and dead notification legacy code

Remove all deprecated Shoutrrr integration artifacts and dead legacy fallback
code from the notification subsystem.

- Remove legacySendFunc field, ErrLegacyFallbackDisabled error, and
  legacyFallbackInvocationError() from notification service
- Delete ShouldUseLegacyFallback() from notification router; simplify
  ShouldUseNotify() by removing now-dead providerEngine parameter
- Remove EngineLegacy engine constant; EngineNotifyV1 is the sole engine
- Remove legacy.fallback_enabled feature flag, retiredLegacyFallbackEnvAliases,
  and parseFlagBool/resolveRetiredLegacyFallback helpers from flags handler
- Remove orphaned EmailRecipients field from NotificationConfig model
- Delete feature_flags_coverage_v2_test.go (tested only the retired flag path)
- Delete security_notifications_test.go.archived (stale archived file)
- Move FIREFOX_E2E_FIXES_SUMMARY.md to docs/implementation/
- Remove root-level scan artifacts tracked in error; add gitignore patterns to
  prevent future tracking of trivy-report.json and related outputs
- Update ARCHITECTURE.instructions.md: Notifications row Shoutrrr → Notify

No functional changes to active notification dispatch or mail delivery.

.github/instructions/ARCHITECTURE.instructions.md

@@ -130,7 +130,7 @@ graph TB
 | **WebSocket** | gorilla/websocket | Latest | Real-time log streaming |
 | **Crypto** | golang.org/x/crypto | Latest | Password hashing, encryption |
 | **Metrics** | Prometheus Client | Latest | Application metrics |
-| **Notifications** | Shoutrrr | Latest | Multi-platform alerts |
+| **Notifications** | Notify | Latest | Multi-platform alerts |
 | **Docker Client** | Docker SDK | Latest | Container discovery |
 | **Logging** | Logrus + Lumberjack | Latest | Structured logging with rotation |
 
@@ -1263,8 +1263,8 @@ docker exec charon /app/scripts/restore-backup.sh \
    - Future: Dynamic plugin loading for custom providers
 
 2. **Notification Channels:**
-   - Shoutrrr provides 40+ channels (Discord, Slack, Email, etc.)
-   - Custom channels via Shoutrrr service URLs
+   - Notify provides multi-platform channels (Discord, Slack, Gotify, etc.)
+   - Provider-based configuration with per-channel feature flags
 
 3. **Authentication Providers:**
    - Current: Local database authentication