[BUG]: currentStreak and lastActivityDate are never updated — Streak5 badge unobtainable, "Experts Online" filter broken

### Bug Description

Two `User` fields are read by features but never written anywhere in the backend, so the features that depend on them are effectively dead.

**`CurrentStreak`** (`models/user.go:32`)
- Initialized to `0` at signup (`auth.go:98`, `auth.go:189`)
- **Read** by the Streak5 badge check: `if user.CurrentStreak >= 5` in `gamification_controller.go:349` and `debatevsbot_controller.go:489`
- Displayed on profile (`profile_controller.go:120`, `:274`)
- **Never incremented** — no `$inc`/`$set` on `currentStreak` exists anywhere in the codebase.
- **Result:** streak is permanently `0`, so `>= 5` is never true → the **Streak5 badge can never be earned**, even though it's defined in code and shown in the UI.

**`LastActivityDate`** (`models/user.go:33`)
- **Read** by the "Experts Online" stat: `leaderboard.go:182` filters `{"lastActivityDate": {"$gte": activeThreshold}}` (active within last 30 min)
- Displayed on profile (`profile_controller.go:122`)
- **Never written** — not even initialized at signup.
- **Result:** `lastActivityDate` is the zero-value `time.Time` for every user. The leaderboard query is `lastActivityDate >= threshold OR updatedAt >= threshold`, so that first clause never matches and the whole stat silently falls back to `updatedAt` — which changes on any profile update, not on actual debate activity. **"Experts Online" measures the wrong thing.**

In short: the reads are wired up, the writes were never implemented. Both fields need to be updated when a user is active (e.g. on completing a debate / meaningful activity) — incrementing/resetting `currentStreak` based on day boundaries, and setting `lastActivityDate = now`.

### Steps to Reproduce

1. Create an account and use the app actively over several days (complete debates, etc.).
2. Check your profile — `currentStreak` stays 0 regardless of daily activity.
3. Reach any amount of activity — the Streak5 badge is never awarded because `CurrentStreak` never rises above 0.
4. Check the "Experts Online" count on the leaderboard — it does not reflect `lastActivityDate` (that field is never set); it only ever reflects `updatedAt`.
5. Search the backend: `grep -rn "currentStreak\|lastActivityDate" backend --include=*.go` shows reads and a signup-time `0`, but no update logic.

### Logs and Screenshots

$ grep -rn "currentStreak\|CurrentStreak" backend --include=*.go
backend/controllers/auth.go:98: CurrentStreak: 0,
backend/controllers/auth.go:189: CurrentStreak: 0,
backend/controllers/gamification_controller.go:349: if user.CurrentStreak >= 5 && ...
backend/controllers/debatevsbot_controller.go:489: if user.CurrentStreak >= 5 && ...
backend/controllers/profile_controller.go:120,274: "currentStreak": user.CurrentStreak
backend/models/user.go:32: CurrentStreak int ...
# → read + init-to-0 only; never incremented

$ grep -rn "lastActivityDate\|LastActivityDate" backend --include=*.go
backend/controllers/leaderboard.go:182: {"lastActivityDate": bson.M{"$gte": activeThreshold}}
backend/controllers/profile_controller.go:122: "lastActivityAt": user.LastActivityDate
backend/models/user.go:33: LastActivityDate time.Time ...
# → read only; never written

### Environment Details

- Files: backend/models/user.go (fields), backend/controllers/auth.go (init), gamification_controller.go & debatevsbot_controller.go (Streak5 read), leaderboard.go (active filter read), profile_controller.go (display)
- Backend: Go / MongoDB
- Branch: main
- Note: purely a missing-write-path bug; no schema change needed, the fields already exist — they just need to be updated on user activity

### Impact

Medium - Feature works but has issues

### Code of Conduct

- [x] I have joined the [Discord server](https://discord.gg/hjUhu33uAn) and will post updates there
- [x] I have searched existing issues to avoid duplicates