Tiles can't be marked, it always shows an error "Friend has already been submitted"

### Problem
Submitting any tile in a game always fails with "Friend has already been submitted", even on fresh, never-used tiles. Tiles never get marked and points never accumulate.

### Steps to reproduce
1. Create a game (user A), join with a second account (user B), start the game
2. Open any tile and add a photo
3. Enter friend B's exact name and code, tap "VERIFY AND MARK TILE"
4. The "Friend has already been submitted" error always appears, whatever the tile

### Root cause
In `app/routes/game.py`, `friend_already_submitted` holds a SQLAlchemy `Query` object rather than the query result. A `Query` object is always truthy, so that check raises every time before the tile is saved. `tile.friend_id` is also never set, so the duplicate-friend check wouldn't catch anything regardless.

### Expected behaviour
- First submission marks the tile and updates points
- Reusing the same friend is rejected
- Submitting the same tile twice is rejected

Start in app/routes/game.py and inspect the friend_already_submitted check and the tile-saving path. Verify that the duplicate-friend query is evaluated as a result and that tile.friend_id is set before saving. Done means a first submission marks the tile and updates points, reusing a friend is rejected, and submitting the same tile twice is rejected.

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