fix: backfill dividend investment_amount_cents and enforce NOT NULL

[WeberG619]

Closes #1596

Problem

Historical dividend records have NULL for investment_amount_cents, causing "N/A" to appear in the "Investment amount" column on /equity/dividends and /equity/dividend_rounds/round/:id pages.

The column was added as nullable in PR #796, but existing dividend records were never backfilled.

Solution

1. One-time backfill script (Onetime::BackfillDividendInvestmentAmounts)

Follows the existing Onetime:: service pattern (like PopulateCompanyInvestorEntities). Run via Rails console before deploying the migration:

Onetime::BackfillDividendInvestmentAmounts.new.perform

Backfill strategy:

• Share-based dividends (number_of_shares IS NOT NULL): Uses company_investor.investment_amount_in_cents
• SAFE/convertible dividends (number_of_shares IS NULL): Uses SUM(convertible_securities.principal_value_in_cents) for that investor
• Fallback: Sets remaining NULL values to 0

2. Migration to enforce NOT NULL

After backfill, adds NOT NULL constraint to dividends.investment_amount_cents. Includes a safety net that sets any remaining NULL values to 0 before applying the constraint.

3. Model validation

Adds validates :investment_amount_cents, presence: true, numericality: { only_integer: true, greater_than_or_equal_to: 0 } to the Dividend model, matching the pattern used on CompanyInvestorEntity.

4. Frontend cleanup

• Removes "N/A" fallback on /equity/dividends page
• Removes conditional hasInvestmentAmounts check on /equity/dividend_rounds/round/:id — the "Investment amount" column is now always shown
• Updates Drizzle schema to mark investmentAmountCents as .notNull()

Deployment steps

1. Deploy the backfill script
2. Run Onetime::BackfillDividendInvestmentAmounts.new.perform in production console
3. Deploy the migration (rails db:migrate)

AI Disclosure

I have used Claude Code (Opus 4.6) to investigate the codebase and implement the fix.

[WeberG619]

CI isn't triggering for the latest push. Could a maintainer please approve the workflow run? The latest commit fixes the remaining 3 test failures (factory default for investment_amount_cents was trying to compute nil * 1000 when number_of_shares is nil for SAFE-based dividends).