feat(backend): implement missing plan management & beneficiary claim endpoints (#1051)

* feat(backend): implement missing plan management & beneficiary claim endpoints

Closes #1034

Added five missing API endpoints that the frontend PlansAPI client
(frontend/app/lib/api/plans.ts) expected but were absent from the
Axum router in backend/src/api.rs:

- GET  /api/plans/{id}              -> get_plan_by_id
- GET  /api/plans/due-for-claim     -> get_plans_due_for_claim
- GET  /api/plans/due-for-claim/{id}-> get_plan_due_for_claim
- POST /api/plans/{id}/claim        -> claim_plan
- POST /api/plans/{id}/cancel       -> cancel_plan

New request/query structs added to api.rs:
  DueForClaimQuery    - wallet_address / page / limit query params
  ClaimPlanRequest    - beneficiary_email + two_fa_code
  CancelPlanRequest   - optional signed_transaction XDR

Handler details:
  get_plan_by_id: queries plans + beneficiaries by UUID, returns
    wrapped { data } response with live yield projection.

  get_plans_due_for_claim: paginated list of plans whose status is
    TRIGGERED or CLAIMABLE; optionally scoped to a beneficiary wallet
    address via ?wallet_address=.

  get_plan_due_for_claim: single-plan variant of the above; returns
    404 when the plan exists but is not yet claimable.

  claim_plan: validates 2FA code format, enforces TRIGGERED/CLAIMABLE
    guard, checks KYC approval for every fiat-configured beneficiary,
    then opens a DB transaction to insert pending payout rows (one per
    beneficiary, shares computed via allocation_bps) and advances the
    plan status to CLAIMABLE.

  cancel_plan: guards against re-cancelling or cancelling PAID_OUT
    plans; optionally validates and submits a signed XDR to Stellar
    before writing; sets is_active = false and status = DEACTIVATED
    in the DB and invalidates the plan cache.

Both claim_plan and cancel_plan are placed on the signature-auth
router so the existing ed25519 middleware protects them. The
due-for-claim and get-by-id routes are public (no auth required)
consistent with the existing GET /api/plans pattern.

* style: apply rustfmt to new plan management & claim endpoints

Author: Creed1759