Editing plugin blocks in the config file (M5 part 2b-1)

[txoof-bot]

Part of #238 (M5 part 2b-1). Built on #241 (part 2a): merge #241 first; this PR then moves to main by itself.

What changed

The code the web interface uses to change [[plugin]] blocks in the config file. The pages that use it come in part 2b-2 (next PR).

• New src/paperpi/web/config_file.py: move a block up or down, remove it, switch it on or off, add a new one at the end of the plugin list.
• It works on the file's text, one block at a time, not with tomlkit. tomlkit counts the comment lines just above a [[plugin]] line as part of the block before it, so they would move with the wrong plugin. Here those comments (when a blank line is above them) belong to the block below them, move with it, and are removed with it.
• Every change is read back with Python's TOML reader and compared with the old settings plus the change. A file written in a way this code doesn't understand is never saved wrongly; it gives the message "change this in the config file itself".
• Each change names its block by place and name, so a block that was moved or renamed by hand meanwhile is not changed by mistake.
• Saving keeps the file's permissions (and owner when run as root), follows a link to the real file, refuses a file larger than PaperPi reads, and holds one lock from read to save. The password save (save_password_hash) now uses the same read, save and lock, so a password change and a plugin change at the same moment can't undo each other.
• Lines are split only at \n, like TOML (Python's splitlines() also splits at U+2028 and others). Windows line endings are kept. Plugin names now also refuse U+0085, U+2028 and U+2029, which Python counts as line breaks. The config check's line numbers use the same rule.
• docs/decisions/config-format.md: note on why plugin blocks are changed as text.

Choices I made without asking:

• A comment on the same line as enabled = is lost when a plugin is switched on or off (switching on removes the line, since the file only holds non-defaults).
• Moving the last block can add one blank line at the end of the file (it does not keep growing).

Area

web (src/paperpi/web/), core (src/paperpi/plugin.py name check, src/paperpi/config.py line numbers), docs.

Tests

New tests/test_web_config_file.py (16 tests): comments move with their block, [plugin.x] parts move too, other parts ([web]) stay, edge of the list, changed meanwhile, unusual headers refused, enabled inside a string or a [plugin.x] part is not the switch, Windows line endings, no last line break, U+2028 inside values, size limit, permissions. Name check test in tests/test_example.py.

• uv run pytest passes (1139 tests on this branch)
• uv run ruff check . passes
• Hardware tests: not needed (no display code)

Review

• Review agents' comments answered: four reviews (code quality, tests, security, docs) ran locally before the first push; their fixes are the second commit ("Review fixes: ..."). Nothing left open.

Only txoof approves and merges this PR.

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