Let an expense's payer invite people

[aquasolterra]

An expense's payer can now mark people they paid for as invited: those people owe nothing back for it. Say Ann pays 90 for Ann, Bo and Cy, and Bo is her guest. Then only Cy owes Ann 30, and the cost doesn't have to be split in two.

Model and math

• New column CostBeneficiaries.invited (schema v38, addColumn guarded by from >= 10). The flag is set per beneficiary, not per cost.
• computeTripStats takes invitedByCost. The guest keeps their share in shareMinor, and totals and categories don't change. The share goes into the guest's invitedMinor and the payer's hostedMinor, and borneMinor = share − invited + hosted feeds netMinor. mergeTripStats pools both fields.
• An invitation only counts on an expense with a payer, for someone other than the payer, on an explicit split (not the participant fallback). The stats ignore it anywhere else, and CostController and the importer never store it there.

UI

• Cost form: tapping a "paid for" chip toggles the invitation; invited people show the volunteer_activism icon. A tristate "Invited by {payer}" box invites everyone or clears all invitations.
• Statistics, balances view: new lines "invited to X" and "invited others to X". The reimbursement line's "own cost" now counts invitations too.

Elsewhere

• Routines: the flag is copied with a routine's fare.
• .tpt: exported as v6 only when the trip uses the flag, because an older app would show the guest owing their share. invited is written only when non-empty.
• PDF: no change. It prints neither splits nor balances.

Tests

• Stats unit tests, DAO tests, routine copy, bundle round trip plus v6 stamping, import sanitizing, the v37→v38 migration, and form and stats-screen widget tests.
• 13 older migration tests now create cost_beneficiaries in their hand-written schemas; every real database from v10 onward has that table. The v37 test also drops the new column.

CHANGELOG, docs/features.md and AGENTS.md are updated.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 90.36145% with 16 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
lib/features/costs/application/cost_providers.dart	61.11%	7 Missing ⚠️
...b/features/costs/presentation/cost_form_sheet.dart	86.48%	5 Missing ⚠️
lib/data/repositories/trip_repository.dart	66.66%	2 Missing ⚠️
lib/data/database/tables.dart	0.00%	1 Missing ⚠️
lib/features/costs/trip_stats.dart	97.22%	1 Missing ⚠️

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