refactor(telemetry): simplify by eliminating TelemetryFacade

**Problem:**
The telemetry system had unnecessary abstraction layers and bad practices
from the last 3 commits introducing the gateway implementation:
- TelemetryFacade class wrapper around emit() function
- String literals instead of SignalType enum
- Dictionary mapping enum → string instead of enum → enum
- Unnecessary ENTERPRISE_TELEMETRY_GATEWAY_ENABLED feature flag
- Duplicate guard checks scattered across files
- Non-thread-safe TelemetryGateway singleton pattern
- Missing guard in ops_trace_task.py causing RuntimeError spam

**Solution:**
1. Deleted TelemetryFacade - replaced with thin emit() function in core/telemetry/__init__.py
2. Added SignalType enum ('trace' | 'metric_log') to enterprise/telemetry/contracts.py
3. Replaced CASE_TO_TRACE_TASK_NAME dict with CASE_TO_TRACE_TASK: dict[TelemetryCase, TraceTaskName]
4. Deleted is_gateway_enabled() and _emit_legacy() - using existing ENTERPRISE_ENABLED + ENTERPRISE_TELEMETRY_ENABLED instead
5. Extracted _should_drop_ee_only_event() helper to eliminate duplicate checks
6. Moved TelemetryGateway singleton to ext_enterprise_telemetry.py:
   - Init once in init_app() for thread-safety
   - Access via get_gateway() function
7. Re-added guard to ops_trace_task.py to prevent RuntimeError when EE=OFF but CE tracing enabled
8. Updated 11 caller files to import 'emit as telemetry_emit' instead of 'TelemetryFacade'

**Result:**
- 322 net lines deleted (533 removed, 211 added)
- All 91 tests pass
- Thread-safe singleton pattern
- Cleaner API surface: from TelemetryFacade.emit() to telemetry_emit()
- Proper enum usage throughout
- No RuntimeError spam in EE=OFF + CE=ON scenario

api/core/moderation/input_moderation.py

@@ -7,7 +7,8 @@ from core.moderation.base import ModerationAction, ModerationError
 from core.moderation.factory import ModerationFactory
 from core.ops.ops_trace_manager import TraceQueueManager
 from core.ops.utils import measure_time
-from core.telemetry import TelemetryContext, TelemetryEvent, TelemetryFacade, TraceTaskName
+from core.telemetry import TelemetryContext, TelemetryEvent, TraceTaskName
+from core.telemetry import emit as telemetry_emit
 
 logger = logging.getLogger(__name__)
 
@@ -49,7 +50,7 @@ class InputModeration:
             moderation_result = moderation_factory.moderation_for_inputs(inputs, query)
 
         if trace_manager:
-            TelemetryFacade.emit(
+            telemetry_emit(
                 TelemetryEvent(
                     name=TraceTaskName.MODERATION_TRACE,
                     context=TelemetryContext(tenant_id=tenant_id, app_id=app_id),