feat: add TypeORM migrations for webhook_subscriptions, soroban_event_logs and index

[silifatzoffun74-art]

• Add webhook_subscriptions table migration with user_id FK to users(id) (ON DELETE CASCADE), url, secret, event_types (jsonb), active, timestamps, plus a WebhookSubscription entity model.
• Add soroban_event_logs table migration with contract_id, ledger_sequence (bigint), topic, tx_hash, payload (jsonb), processed, created_at, plus a SorobanEventLog entity model.
• Add composite index IDX_SOROBAN_EVENTS_CONTRACT_LEDGER on soroban_event_logs (contract_id, ledger_sequence DESC) in its own migration, ordered after the table-creation migration.

Migration timestamps are ordered so TypeORM runs them in the correct sequence: webhook_subscriptions table → soroban_event_logs table → composite index.

Test plan

• npx tsc --noEmit — no errors
• npx eslint src/**/*.ts — clean (also runs via pre-commit hook)
• npx jest — 400/401 passing; 1 pre-existing failure in tests/validate-invoice-for-publish.test.ts unrelated to this change (invoice validation error ordering), left untouched
• Migration up/down verified against a live Postgres instance (not run — no local DB available in this environment; migrations follow the exact raw-SQL style of existing migrations in src/migrations)

Closes #168
Closes #167
Closes #166

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