feat: store full datetime in sleep_segments JSONB

Enhanced sleep_segments data structure:
- start: ISO datetime (2026-03-21T22:30:00) instead of HH:MM
- end: ISO datetime (2026-03-21T23:15:00) - NEW
- phase: sleep phase type
- duration_min: duration in minutes

Benefits:
- Exact timestamp for each segment (no date ambiguity)
- Can reconstruct complete sleep timeline
- Enables precise cycle analysis
- Handles midnight crossings correctly

Example:
[
  {"phase": "light", "start": "2026-03-21T22:30:00", "end": "2026-03-21T23:15:00", "duration_min": 45},
  {"phase": "deep", "start": "2026-03-21T23:15:00", "end": "2026-03-22T00:30:00", "duration_min": 75}
]

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/routers/sleep.py

@@ -545,11 +545,12 @@ async def import_apple_health_sleep(
                 night['awake_minutes']
             )
 
-            # Prepare JSONB segments
+            # Prepare JSONB segments with full datetime
             sleep_segments = [
                 {
                     'phase': seg['phase'],
-                    'start': seg['start'].strftime('%H:%M'),
+                    'start': seg['start'].isoformat(),  # Full datetime: 2026-03-21T22:30:00
+                    'end': seg['end'].isoformat(),      # Full datetime: 2026-03-21T23:15:00
                     'duration_min': seg['duration_min']
                 }
                 for seg in night['segments']