fix(files): report new files as new in the files:scan summary

[seenickcode]

Summary

occ files:scan has reported every new file as "Updated" (never "New")
since the summary table was introduced in 292c0e5.

Root cause: the addToCache listener decides between NodeAddedToCache and
FileCacheUpdated by checking if ($fileId), but Cache\Scanner emits
$fileId = -1 for newly inserted entries, which is truthy. As a result,
the NodeAddedToCache branch was unreachable.

What it does

• Checks for the actual -1 sentinel instead, so newly inserted entries
  dispatch NodeAddedToCache and existing entries dispatch
  FileCacheUpdated
• Adds a regression test asserting that a first scan dispatches
  NodeAddedToCache and a re-scan of a modified file dispatches
  FileCacheUpdated

How I verified

Setup: Nextcloud master, MariaDB, an S3 (MinIO) external storage mount with
100 files in 35 folders, mounted at /legacy for user demo01.

Before each scan I cleared the mount's rows from the file cache so every
entry the scan finds is guaranteed new:

DELETE FROM oc_filecache WHERE storage = <numeric_id of the mount>;

On master — all 135 entries were just deleted, so all of them are new,
yet the summary reports none:

$ occ files:scan demo01 --path=/demo01/files/legacy
Starting scan for user 1 out of 1 (demo01)
+---------+-------+-----+---------+---------+--------+--------------+
| Folders | Files | New | Updated | Removed | Errors | Elapsed time |
+---------+-------+-----+---------+---------+--------+--------------+
| 35      | 100   | 0   | 135     | 0       | 0      | 00:00:00     |
+---------+-------+-----+---------+---------+--------+--------------+

On this branch — identical steps:

$ occ files:scan demo01 --path=/demo01/files/legacy
Starting scan for user 1 out of 1 (demo01)
+---------+-------+-----+---------+---------+--------+--------------+
| Folders | Files | New | Updated | Removed | Errors | Elapsed time |
+---------+-------+-----+---------+---------+--------+--------------+
| 35      | 100   | 135 | 0       | 0       | 0      | 00:00:00     |
+---------+-------+-----+---------+---------+--------+--------------+

The included regression test also fails against master and passes with the
fix (first scan must dispatch NodeAddedToCache, re-scan of a modified
file must dispatch FileCacheUpdated).

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI