Add veganversary to profile and weekly digest

[timkwist]

Context

Members already have an optional birthday on onboarding and profile (birthday_month / birthday_day / birthday_year, plus show_birthday). There is no veganversary (the date someone went vegan).

The weekly digest (send_weekly_digest → emails/weekly_digest.html) is currently upcoming events only. PDA should celebrate people whose veganversary falls in that week.

There is no public self-signup — accounts are created by admins, then members set profile fields during onboarding and can edit them later.

What needs to happen

1. Add an optional veganversary field on onboarding and the member's own profile edit.
   • Empty is allowed (field is optional).
   • Minimum if anything is entered: month and year. Month-only and year-only are invalid.
   • Day is optional.
   • Helper copy: "the exact date isn't required, but please let us know at least the month and year!"
   • Nearby line: "if you enter your veganversary, we'll celebrate you by name when your veganversary comes up!"
2. Include a veganversary section in each weekly digest for members whose veganversary falls in that digest week, copy along the lines of: "if you see these people, tell them happy veganversary!"
3. Split that section by number of years (1 year, 2 years, …) based on the stored year vs the digest year. Every stored veganversary has a year, so grouping is always possible.

Acceptance criteria

• Members can optionally set a veganversary on onboarding and on their profile
• Day is optional; month-only and year-only are rejected; month and year are both required if anything is set
• Field copy includes "the exact date isn't required, but please let us know at least the month and year!"
• Nearby copy: "if you enter your veganversary, we'll celebrate you by name when your veganversary comes up!"
• Each weekly digest includes a veganversary section listing people whose veganversary falls in that week
• The section uses copy like "if you see these people, tell them happy veganversary!"
• The section is split by number of years