Windows task execution fix and refactored system task registry

[elcreator]

#2428 should be merged first

Fix 1 — Composer detection on Windows

Turned out to need three parts, not one. The third only surfaced because I tested instead of assuming:

• shellCommandExists() — command -v X >/dev/null 2>&1 → branches to where X >NUL 2>NUL on Windows. Verified: exit 0 for a real command, 1 for a bogus one.
• composerBinaryCandidates() — added %ProgramData%\ComposerSetup\bin\composer.bat|.exe, %APPDATA%\Composer\composer.bat|.exe, and the AppData/Roaming form of each home directory. composer.phar is deliberately excluded: it
  needs php in front, and this list feeds a directly-executed command.
• isExecutableFile() — new, and the part I'd have missed. is_executable() returns false on Windows even for a genuine composer.bat (verified against the real C:\ProgramData\ComposerSetup\bin\composer.bat: is_file()=true,
  is_executable()=false). Without this, every Windows candidate would be rejected no matter which paths I added, and fix 2 of 3 would have been silently useless.

One design change mid-way: I first made composerBinaryCandidates() switch on platform, which broke the existing test site updater checks user local composer path as a fallback (it asserts the POSIX path is present, and passed
on Windows before). Candidates are filtered by existence anyway, so I made the list a superset instead — an impossible entry costs one is_file() call, and a platform branch is just a new way to guess wrong under WSL.

Fix 2 — PostgreSQL backup on Windows

PGPASSWORD=… pg_dump … >> file via exec() replaced with Symfony Process taking an argument list and ['PGPASSWORD' => $password] as env. Verified the old form fails here: 'PGPASSWORD' is not recognized as an internal or
external command.

I kept it streaming. The shell redirect it replaces streamed to disk; getOutput() would have buffered an entire database dump in memory, which would have been a regression on exactly the databases worth backing up. The run
callback writes each chunk to an append handle. Also added clearstatcache() before the filesize() check, which the shell version didn't need.