Add memory weight: critical, normal and low #294

See #290, section 2.

Work

• Weight on memories (critical, normal default, low), settable through memory_store, the web editor and batch store; changing weight of shared or public memories follows the usual review rules.
• Recall reserves part of its budget for critical memories and lists them first; low memories are not listed in recall but stay searchable.
• Hygiene score: add the "too many critical memories" finding and weight the stale-bag finding later by child weight.
• Web: weight visible in list and detail, filterable.

Acceptance

• A project whose normal memories exceed the budget still shows every critical memory in recall.
• Low memories never appear in recall and are found by item_search.
• Tests for weight changes through proposals.