Fix: ETA calculations were wildly optimistic
ETAs were computed using distinct watching days as the denominator instead of calendar days since first watch. This assumed you watch every single day, making achievements look much closer than they really are.
Example: 208/1000 late-night sessions, user has been on Jellyfin ~365 days but only watched on 67 of them:
- Old: rate = 208/67 = 3.1/day → ETA = 256 days (wrong — assumes daily watching)
- New: rate = 208/365 = 0.57/day → ETA = 1,390 days (correct — reflects real pace)
Now uses the earliest entry in the user's WatchDates set to compute calendar days elapsed, so the rate reflects actual real-world pace including off-days.