feature: Add this_day filter

[d-jankowski]

Description

Adds a new this_day() CEL filter function that matches memos created on today's month-day in any year. Supports all three database dialects (SQLite, MySQL, PostgreSQL).

Changes

• Add ThisDayCondition IR node in the filter pipeline.
• Register this_day as a zero-arg boolean CEL function in the memo schema.
• Parse this_day() calls into ThisDayCondition in the filter parser.
• Render dialect-specific SQL comparing created_ts month-day against current date for SQLite, MySQL, and PostgreSQL.
• Add CreatedTs builder method to test helpers for setting custom creation timestamps.
• Add tests covering basic matching, combined filters, no-match, and negation.

Usage

this_day()                          # memos created on today's month-day in any year
this_day() && tag in ["work"]       # combine with other filters
!this_day()                         # exclude today's date matches

Related Issue

Resolves #5633

Testing

Manual

1. Create a new shortcut with this_day() in the Filter field and save it.
2. Create several memos, then edit their creation dates to fall on today's month-day but in different years (e.g., 2024, 2023).
3. Create additional memos with creation dates that do not match today's month-day.
4. Apply the shortcut. Only the memos from step 2 should appear.

Automated

• TestMemoFilterThisDay - matches today + same day last year, excludes different day.
• TestMemoFilterThisDayCombinedWithOtherFilters - combines with tag filter.
• TestMemoFilterThisDayNoMatches - only different-day memos, expects 0 results.
• TestMemoFilterThisDayNegated - verifies !this_day() negation.