feat(rules): apply a new rule to existing transactions on creation

[tassionoronha]

Summary

Follow-up to the rule-form UX work on #306. Previously, creating a rule only saved it — existing transactions stayed untouched until the user manually clicked Reapply all. This was an invisible gap, especially when creating rules in bulk.

Now, creating a rule immediately applies it to existing transactions and the success toast reports the result with a single combined message:

Rule created and applied to 4 transactions

(falling back to plain "Rule created" when nothing matched).

Behavior

The auto-apply is non-destructive by design:

• A transaction that already has a category keeps it — same semantics used when new transactions arrive via sync/import, so creating a rule never silently overwrites your manual categorizations.
• Payee / notes / ignore actions still apply on a match.
• Only transactions whose fields actually change are counted, so the toast number is honest.

For a full reset that re-runs every rule and overwrites, Reapply all is still there and unchanged.

Changes

• backend: new rule_service.apply_single_rule(); POST /api/rules now returns RuleCreateResponse (= RuleRead + applied_count).
• frontend: combined toast in createMutation.onSuccess, plus invalidateFinancialQueries when count > 0 so transactions/reports refresh.
• i18n: rules.createdAndApplied added to en, pt-BR, es, it, pl.
• tests: 3 new backend tests — applies & counts, no-match → 0, and non-destructive on an already-categorized transaction.

Testing

• Backend rule suites pass (test_new_rules_api, test_rule_service, test_rule_engine) — 38 tests including the 3 new ones. Frontend tsc -b + eslint clean.
• Verified live in the running app (Docker + Chrome): created a rule matching 4 uncategorized transactions → toast showed "Rule created and applied to 4 transactions"; the transactions were categorized without an explicit Reapply. An already-categorized transaction was confirmed untouched. Test rule and transaction changes were reverted afterward.