Move code shared by the collectors into one module

The Veeam, MARS, WSB and VB365 collectors each carried identical copies
of logging, ISO time conversion, property lookup, posting to the API,
config loading, TLS setup, the sent-runs state file and the 401 check,
and their installers repeated the same key prompt, locked-down config,
file copy and scheduled-task registration. These now live in
collectors/common/BackupDashboard.Collector.psm1:

- Initialize-Collector / Test-RunSent / Send-JobReport /
  Test-AuthRejected / Complete-Collector for collector runs
- Write-Log, Get-ErrorText, ConvertTo-IsoTime, Get-FirstValue
- Install-CollectorTask for the shared half of each installer

Collectors find the module next to themselves (installed) or in
..\common (repo); installers copy it into Program Files beside the
collector. The diagnostic scripts stay standalone single files.

No behavior change: a harness ran each collector against its mock
(12 scenarios: dry run, two real runs against a stub API, state file,
logs, exit codes, rejected key, en-GB, backfill) before and after, and
all 84 outputs are identical. Installers, run with stand-ins for the
Windows-only commands, write identical configs and tasks; the only
differences are the module being copied and the summary line naming
the shell's path.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

collectors/mars/README.md

@@ -18,7 +18,8 @@ targets and which every supported Windows Server has.
    ```bash
    python manage.py create-key mars-acme-fs01
    ```
-2. Copy this folder to the server and run:
+2. Copy the whole `collectors` folder to the server (the installer needs `common`
+   as well as this folder), then from `collectors\mars` run:
    ```powershell
    .\Install-MarsCollector.ps1 -ApiUrl https://backups.example.com -Tenant "Acme Corp"
    ```
@@ -33,8 +34,9 @@ targets and which every supported Windows Server has.
    powershell.exe -File 'C:\Program Files\BackupDashboard\Send-MarsBackupStatus.ps1' -LookbackHours 720
    ```
 
-It can be installed alongside the Veeam collector: it uses its own config, state, log and task.
-Re-run the installer to change settings or update the script.
+It can be installed alongside the other collectors: it uses its own config, state, log and task.
+Re-run the installer to change settings or update the script. It also installs
+`BackupDashboard.Collector.psm1`, the code shared by all collectors, next to the script.
 
 ## What it sends