Backfill facilitator affiliation start dates from legacy member_since

Problem

Facilitator start dates now come from affiliation records, but the profile/decorator still falls back to the legacy member_since column when a Facilitator affiliation has no start_date:

• PersonDecorator#facilitator_since_date → affiliations.facilitators.minimum(:start_date) || member_since (app/decorators/person_decorator.rb).

So where a facilitator's affiliations are missing start dates, the displayed "Facilitator since" is driven by member_since rather than the affiliation data. The edit form already flags the mismatch (member_since_earlier_than_facilitator_affiliations? / member_since_differs_from_facilitator_affiliations?), which shows these two sources disagree in production data.

The result: the affiliation records and the profile displays can be out of sync, and the "Affiliated since" grey note (added in #2280) only reads correctly once affiliation dates are trustworthy.

Goal

Make the affiliation data the single source of truth so member_since is no longer needed as a fallback for facilitators.

Scope / questions to settle

• Backfill direction: for facilitators whose Facilitator affiliation(s) have a blank start_date, populate it from member_since (the earliest sensible value).
• Conflicts: decide what to do when member_since is earlier than or differs from an existing affiliation start date — earlier likely means an affiliation record is missing that stretch; a plain difference may just be legacy noise. Don't clobber real affiliation start dates.
• Idempotent migration/rake task with a dry-run report of what would change (counts of: blank→filled, member_since earlier than all facilitator affiliations, member_since differs), reviewable before running against prod.
• After backfill: confirm the member_since fallback in facilitator_since_date can be removed (or is only hit by legacy edge cases), and drop the form warnings once the data is clean.

Notes

• Related display work: #2280 (affiliated-since grey note).
• Same pattern likely applies to org-level program dates (OrganizationDecorator#program_since_date) and the org start_date fallback — worth checking in the same pass.

Start with app/decorators/person_decorator.rb and the facilitator_since_date fallback, then review the affiliation start-date checks named in the issue. Define the conflict handling and inspect the related organization fallback before implementing an idempotent migration or rake task with a dry-run report. Done means missing dates are backfilled without clobbering existing dates, the report is reviewable, and the fallback and warnings are reassessed.