fix(worker): graceful skip when Servers or Messages data is absent

[Androz2091]

Same fallback bug that PR #82 fixed for the analytics path, but for Servers and Messages.

A user reported UNKNOWN_ERROR for package `4aa23a2a6ea2f52ba9016a723143b792`. CloudWatch:

```
File "/app/tasks.py", line 466, in read_analytics_file
server_content = zip.open(server_path)
KeyError: "There is no item named 'Servers/index.json' in the archive"
```

The user un-ticked Servers in Discord's data-request form. The old code:

```py
servers_root = find_servers_root(namelist)
if not servers_root:
server_path = 'Servers/index.json' # blind fallback
if server_path not in namelist and 'servers/index.json' in namelist:
server_path = 'servers/index.json'
else:
server_path = f'{servers_root}/index.json'

server_content = zip.open(server_path) # KeyError if neither exists
```

If `find_servers_root` returns None and neither lowercase fallback matches the existence check (the existing logic only swaps to lowercase if the canonical path isn't present and lowercase is — so if both are absent, `server_path` ends up as the literal `'Servers/index.json'`), `zip.open` blows up.

Same exact pattern in the Messages block right below.

Fix

Make the path resolution defensive — only assign a path that actually exists in `zip.namelist()`; if nothing matches, set None and skip the read block, marking `is_partial=True`. The rest of the worker tolerates empty servers / empty channels lists (the message-CSV walker filters by namelist, so it processes 0 channels naturally).

Possible outcomes after this:

Servers?	Messages?	Result
✅	✅	Normal full package
❌	✅	DMs / friends / payments / per-channel stats; server stats N/A. `is_partial=true`
✅	❌	Server list / friends / payments; channel and DM stats N/A. `is_partial=true`
❌	❌	Profile + friends + payments only. `is_partial=true`

Test plan

• Merge → CI deploys.
• Re-submit `4aa23a2a6ea2f52ba9016a723143b792` — expect PROCESSED with `is_partial=true`.
• No regression on packages with both folders present (most common case).