feat(api): auto-complete savings goals when current_amount reaches target_amount

[Salmatcre8]

Summary

Goals had no status field and never transitioned to completed at 100%, so the frontend couldn't distinguish an in-progress goal from a fully-funded one, and there was no trigger point for completion notifications / celebration UI.

Closes #684

What's included

• Migration 040 — adds status (active|completed|archived, default active, CHECK-constrained) and completed_at columns, plus a status index.
• Domain — GoalStatus type + ParseStatus; Status and CompletedAt on SavingsGoal; ErrGoalCompleted.
• Auto-complete — enrichProgress transitions an active goal to completed once progress_pct >= 100, stamps completed_at, and persists the change.
• List filter — GET /api/v1/users/savings-goals?status=active|completed|archived (default returns all). Filtering is applied post-enrichment so a goal that just hit 100% shows up under ?status=completed immediately.
• Immutability — PATCH on a completed goal returns 409 (ErrGoalCompleted); a completed goal may only be archived.
• Repository — reads/writes the new columns; Create defaults status=active.

Acceptance criteria

• Migration adds status (default active) + completed_at
• Auto-transition to completed at progress_pct >= 100
• completed_at set on completion
• ?status=completed returns only completed goals
• PATCH on a completed goal → 409
• Unit test: 99% stays active; 100% transitions to completed
• Integration test: reaching target flips status to completed

Tests

ok  github.com/suncrestlabs/nester/apps/api/internal/handler

Real SavingsGoalService + in-memory repo, driven through the handler: 99% stays active, 100% auto-completes with completed_at, ?status=completed filter, and PATCH completed → 409.

Notes / judgment calls

• Tests live in the handler package (real service + in-memory repo, end-to-end via HTTP) because internal/service's existing *_test.go files (savings_goal_service_test.go, savings_schedule_service_test.go) do not compile on main — leftovers from #689 feat(api): fire push notifications when savings goal crosses #710 / the savings-schedule merge (stale mocks missing UpdateMilestones/CreateVault, uuid-keyed balances, missing constructor args). I deliberately kept this PR from touching those files to avoid conflicts with whatever fixes them; the behaviour is fully covered through the handler instead.
• Pre-existing, unrelated: cmd/api/main.go does not build on main (wsHub missing PushToUser) — untouched here; my internal/... packages build and vet cleanly.
• A completed goal is intentionally immutable except for archiving (status: archived); manual status: completed via PATCH is rejected (completion is automatic).

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[0xDeon]

Merge conflict with main. Rebase onto main and resolve conflicts before this can be merged.