Add sharding to collection files

[twpol]

Currently all files are stored in a single directory, but this will quickly become unmanageable by the file system and application when there are thousands of files

Proposal to add sharding based on the current week, using ISO year + week, e.g. 2025-W14 for the week starting 31th March 2025, since all statistics will necessarily be calculated on week multiples anyway (anything else, like months, leads to weirdness due to month length, week alignment, etc.)