feat :Field-level AES-256-GCM encryption at rest for reversible secrets

[githoboman]

What was implemented
Field-level AES-256-GCM encryption at rest for reversible secrets, applied to webhook signing secrets.
Closes #683

New files
src/lib/encryption.ts — the envelope-encryption helper. encryptField/decryptField using AES-256-GCM. Ciphertext is stored as v1:::: (base64), self-describing and tagged with the key id so rotated-out keys still decrypt old data. Also exposes resolveKeys, isEncrypted, encryptNullable/decryptNullable, and typed DecryptionError/EncryptionKeyError. Fails closed: throws if no key is configured and never returns ciphertext on a decrypt failure.
src/tests/encryption.test.ts — covers round-trip (incl. empty/unicode), tamper detection on ciphertext/tag/IV, no-silent-passthrough, wrong-key rejection, key-id rotation (decrypt under retired key, active = first key, orphaned-kid failure), and config failures (missing key, wrong length, bad JSON, duplicate kid).
docs/field-encryption.md — scheme + zero-downtime key-rotation runbook.
jest.setup.cjs — seeds a test-only FIELD_ENCRYPTION_KEY so existing DB-gated webhook suites keep working.
Edited
src/config/env.ts — added FIELD_ENCRYPTION_KEY / FIELD_ENCRYPTION_KEYS to the schema and a getFieldEncryptionConfig() resolver (read directly from process.env, decoupled from full initEnv() so the helper works in isolation).
src/repositories/webhookSubscriberRepository.ts — encrypts on write (create, upsert, rotateSecret) and decrypts on read (toSubscriber). Reads tolerate legacy plaintext during rollout (decryptSecretColumn), which self-heals to ciphertext on next write. The service layer's reversible secret contract is unchanged — plaintext exists only in memory.
src/tests/webhookSubscriber.rotation.test.ts — raw-DB-column assertions now decrypt before comparing.
.env.example, docs/security-ci.md, jest.config.cjs — config/doc/test wiring.
No package.json changes.

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