feat(telemedicine): TURN rotation, chat-cache isolation, attachment validation, runtime response parsing

[willy-de7]

Summary

Security hardening for the telemedicine relay, chat cache, attachment pipeline
and mobile API response parsing. Four narrowly-scoped, dependency-free modules
(Node crypto / Buffer only, node Jest environment) plus focused unit
suites and contributor docs.

Closes #971
Closes #972
Closes #973
Closes #974

#971 — TURN credential rotation and expiry

src/services/turnCredentialService.ts

• Ephemeral coturn-style credentials: username = <expiryUnix>:<principal>,
  credential = base64(HMAC-SHA1(sharedSecret, username)). No long-lived secret
  reaches a device; a leaked credential dies at its embedded expiry (default TTL
  300s, clamped 30s–24h).
• verifyTurnCredential uses timingSafeEqual and accepts the previous
  secret during a rotation grace window so rotating the shared secret never
  drops an in-flight call.
• TurnCredentialManager caches one live credential, re-issues within a 60s
  refresh skew, coalesces concurrent fetches, and exposes
  recoverExpiredSession() for the "relay rejected the session" path.

#972 — Prevent telemedicine message history leakage on logout

src/services/telemedicineChatVault.ts

• Per-account random 256-bit data key in the secure keystore; cache rows are
  AES-256-GCM ciphertext keyed by account id (chat bodies + attachment metadata).
• logout() / switchAccount() destroy the key first, then purge the
  cache — an interrupted purge still fails closed because leftover ciphertext is
  unreadable.
• readHistory() returns [] when no key is present, so a newly signed-in
  account can never read the previous account's cached history.
• Injectable SecureKeystore / CacheStore seams; InMemoryStore provided for
  tests.

#973 — Attachment malware and content-type validation for chat

src/services/attachmentValidationService.ts

• sniffContentType identifies files by magic bytes; looksExecutable blocks
  PE / ELF / Mach-O / Java class / shell scripts / <script / <?php.
• validateAttachment enforces MAX_ATTACHMENT_BYTES (15 MiB) and an allowlist
  (png/jpeg/gif/webp/pdf/plain), rejects empty files, and quarantines
  declared-vs-detected type mismatches and disguised executables.
• safeDownloadHeaders returns Content-Disposition: attachment,
  X-Content-Type-Options: nosniff,
  Content-Security-Policy: default-src 'none'; sandbox,
  Cache-Control: private, no-store.
• Filenames sanitised (strip path / traversal, [\w.-] only, 128-char cap).

#974 — Runtime response validation for mobile API services

src/services/runtimeValidation.ts

• Tiny schema combinator (v.object/array/string/number/boolean/literal/optional)
  with .parse() → { success, data } | { success:false, issues }; rejects
  null, wrong types, NaN/Infinity, and array/object confusion with
  path-tagged issues.
• parseResponse(schema, data, context) throws a single
  ResponseValidationError carrying every issue instead of letting malformed
  nested JSON crash a screen.
• Wired into telemedicineService.getTelemedicineAvailability; the pattern
  extends to the remaining src/services calls incrementally.

Tests

New unit suites (synthetic data only, no secrets/tokens/health data in
fixtures or logs):

• src/services/__tests__/turnCredentialService.test.ts — issue, TTL clamping,
  HMAC verify/tamper/expiry, rotation grace window, manager caching + refresh +
  expired-session recovery + fetch coalescing.
• src/services/__tests__/telemedicineChatVault.test.ts — encrypted round-trip,
  ciphertext-not-plaintext at rest, [] after logout, account-switch isolation,
  key-before-cache teardown on interrupted purge, idempotent logout.
• src/services/__tests__/attachmentValidationService.test.ts — magic-byte
  sniffing, executable/script detection, filename sanitising, type-spoof and
  disguised-executable quarantine, size/empty limits, safe headers.
• src/services/__tests__/runtimeValidation.test.ts — happy path, malformed
  nested data yields typed issues (no throw), optional handling, type guards,
  parseResponse aggregate error.

Note: node_modules is not installed in this working copy, so the suites were
not executed locally — CI will run them. Crypto primitives (HMAC-SHA1 verify,
AES-256-GCM round-trip) were smoke-tested directly against Node.

Platform

Pure TypeScript, no native modules — iOS and Android behave identically.
Injectable keystore / cache / clock seams keep every flow testable without a
device. No UI changes.

Docs

docs/TELEMEDICINE_SECURITY.md documents all four flows.

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