Fix multiple critical bugs in onboarding training step

This commit addresses all identified bugs and issues in the training code path:

## Critical Fixes:
- Add get_start_time() method to TrainingLogRepository and fix non-existent method call
- Remove duplicate training.started event from API endpoint (trainer publishes the accurate one)
- Add missing progress events for 80-100% range (85%, 92%, 94%) to eliminate progress "dead zone"

## High Priority Fixes:
- Fix division by zero risk in time estimation with double-check and max() safety
- Remove unreachable exception handler in training_operations.py
- Simplify WebSocket token refresh logic to only reconnect on actual user session changes

## Medium Priority Fixes:
- Fix auto-start training effect with useRef to prevent duplicate starts
- Add HTTP polling debounce delay (5s) to prevent race conditions with WebSocket
- Extract all magic numbers to centralized constants files:
  - Backend: services/training/app/core/training_constants.py
  - Frontend: frontend/src/constants/training.ts
- Standardize error logging with exc_info=True on critical errors

## Code Quality Improvements:
- All progress percentages now use named constants
- All timeouts and intervals now use named constants
- Improved code maintainability and readability
- Better separation of concerns

## Files Changed:
- Backend: training_service.py, trainer.py, training_events.py, progress_tracker.py
- Backend: training_operations.py, training_log_repository.py, training_constants.py (new)
- Frontend: training.ts (hooks), MLTrainingStep.tsx, training.ts (constants, new)

All training progress events now properly flow from 0% to 100% with no gaps.

services/training/app/repositories/training_log_repository.py

@@ -329,4 +329,17 @@ class TrainingLogRepository(TrainingBaseRepository):
                 "min_duration_minutes": 0.0,
                 "max_duration_minutes": 0.0,
                 "completed_jobs_with_duration": 0
-            }
+            }
+
+    async def get_start_time(self, job_id: str) -> Optional[datetime]:
+        """Get the start time for a training job"""
+        try:
+            log_entry = await self.get_by_job_id(job_id)
+            if log_entry and log_entry.start_time:
+                return log_entry.start_time
+            return None
+        except Exception as e:
+            logger.error("Failed to get start time",
+                        job_id=job_id,
+                        error=str(e))
+            return None