OAuth Callback Missing Hardened TLS Configuration

## Issue: FINDING-068 - OAuth Callback Missing Hardened TLS Configuration

**Labels:** bug, security, priority:medium, asvs-level:L2

**ASVS Level(s):** [L2-only]

**Description:**

### Summary
The OAuth callback endpoint creates a plain `aiohttp.ClientSession` instead of using `create_secure_session()`, missing TLS 1.2+ enforcement, explicit certificate verification, hostname checking, and secure cipher suite selection. This creates a potential man-in-the-middle vulnerability in the OAuth token exchange flow.

### Details
**Affected Files and Lines:**
- `src/asfquart/generics.py:98` - Plain ClientSession without hardened TLS

The OAuth token exchange uses default TLS settings instead of the application's hardened configuration.

### Recommended Remediation
Use `create_secure_session()` for hardened TLS configuration:

```python
async with util.create_secure_session(
timeout=aiohttp.ClientTimeout(sock_read=15)
) as session:
rv = await session.get(OAUTH_URL_CALLBACK % encoded_code)
```

### Acceptance Criteria
- [ ] Hardened session used
- [ ] TLS 1.2+ enforced
- [ ] Certificate verification enforced
- [ ] Hostname checking enforced
- [ ] Integration test verifies configuration
- [ ] Unit test verifying the fix

### References
- Source reports: L2:1.3.6.md
- Related findings: FINDING-067
- ASVS sections: 1.3.6

### Priority
Medium