`UserProfileFunctions.cs`: Deserialization errors return generic 400 with no structured logging

### Problem

When request body deserialization fails in `UserProfileFunctions`, the handler returns a generic `BadRequest("Invalid request body.")` with no log entry:

**File:** `PluckIt.Functions/Functions/UserProfileFunctions.cs:63`

```csharp
catch (JsonException)
{
return new BadRequestObjectResult("Invalid request body.");
}
```

Operators have no way to see what payload caused the failure, which field was missing, or how often this happens.

### Proposed Fix

Add structured logging before returning:

```csharp
catch (JsonException ex)
{
logger.LogWarning(ex, "Profile deserialization failed for request to {Path}", req.Path);
return new BadRequestObjectResult("Invalid request body.");
}
```

### Functionality Impact

None. Same 400 response to callers, but now logged for operators.