fix(budget): cherry-pick VAT storage semantics fix to beta (#1387)

[steilerDev]

Summary

Cherry-pick of #1387 (hotfix merged to main) back to beta.

• Removes the buggy /1.19 division in direct-mode toPayload — stored plannedAmount now equals exactly what the user entered
• includesVat is now persisted for direct-mode budget lines (was always null), defaulting to true
• Unit-mode save no longer bakes VAT into plannedAmount; VAT is applied at display/calculation time via the new effectivePlannedAmount() shared utility
• DB migration 0031 converts existing unit-mode includes_vat=false lines from VAT-inclusive back to net amounts

Test plan

• Quality Gates pass
• Direct-mode budget line: enter €100 with "price includes VAT" → reloads as €100
• Direct-mode budget line: enter €100 without "price includes VAT" → displays and totals as €119

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