Fix budgets for categories missing from snapshots

[almeidandres]

Problem

A category excluded when a monthly budget snapshot was created has no row in that snapshot. Re-enabling the category and setting its budget then returns “No budget snapshot found for this category and month”.

Fix

Insert a snapshot row when the month already contains snapshot rows but the requested owned category does not. The new row uses the submitted values and falls back to the category's effective budget for omitted fields. Existing rows remain unchanged.

Checks

• CategoryServiceTest: 66 tests, 158 assertions
• Regression test for adding a missing category to an existing snapshot month
• PHP CS Fixer: no changes needed
• Psalm: no errors in CategoryService.php

[otherworld-dev]

Merged with a few changes on top. The new row now starts from the budget the page was already showing for that month, so a period-only save keeps an earlier "no budget" instead of picking up the category's default. Two quick saves that both miss the row no longer show the user the unique index error, and an unknown category keeps the "No budget snapshot found" message rather than the SQL from the mapper. I also reworded the changelog, the adjustment copies every category including excluded ones, so the missing row only happens for a category created after the adjustment was made. Thanks for the fix and the test.