Validate "Excluded topic IDs" and let the field be cleared

[avandenberghe]

Closes #217.

Problem (raised in the customisations team's 3.0.11 review)

• The ACP saved rt_anti_topics only when every comma-separated entry was numeric. Otherwise it silently kept the old value but still showed "Configuration updated".
• An empty field never saved: explode(',', '') gives [''], which isn't numeric, so the list couldn't be cleared (the workaround was 0).

Fix (acp/recenttopics_module.php)

• New normalise_anti_topics(): trims entries, drops empty ones (a trailing comma, a blank field) and duplicates, and turns an empty list into '0' (exclude nothing). It returns null if any entry isn't a whole number.
• main() runs the check right after the form-key check, before any setting is written. Invalid input shows the new RT_ANTI_TOPICS_INVALID error and nothing is saved.
• RT_ANTI_TOPICS_INVALID is translated in all 14 ACP language packs.

Tests

• New tests/acp/anti_topics_test.php: 11 inputs, from valid lists, spacing, duplicates, a trailing comma and an empty field to text, decimals and negatives. All errored before the helper existed.
• New functional tests (CI): invalid input shows the error and leaves the stored value unchanged; clearing the field stores 0.
• tests/tests.md updated. Unit suite: 101 tests, 0 failures, run locally. CI result to follow.
• Not run on the board: the save path needs a logged-in admin session. The functional tests cover it in CI. A stored 0 is what the topic query already treats as "exclude nothing".

This folds into 3.0.12; its changelog line comes with the 3.0.12 changelog update.

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