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Add monthly expense report endpoint

New GET /api/Report?percentageA={0..100} computes pair totals, per-user
share, and the net settlement server-side. Replaces a frontend
calculation that fanned out to four payment/debt total endpoints.

Pair boundary is enforced by construction: the endpoint takes no userId,
the pair is resolved from the JWT, and the service queries only users in
that pair.

Also extracts the duplicated GetCallerPair / IsAllowedForUser helpers
from PaymentController and DebtController into PairAuthExtensions, now
shared by all three controllers.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>