DC-046 DC-047 pluggable auth providers + email magic link

Pluggable AuthProvider framework for any future auth method (OIDC, SAML,
passkeys) to plug in without touching the auth path again. Two
implementations ship:

  * TOTP — refactored from routes/auth/totp.js into src/auth/providers/totp.js
    as one AuthProvider impl. Legacy /api/v1/totp/* routes stay mounted for
    back-compat; new /api/v1/auth/login/totp/* routes use the new shape.

  * EmailMagicLink — src/auth/providers/email.js. Initiate issues a 32-byte
    base64url token, stores its SHA-256 hash in data/email-tokens.json
    (atomic lockfile-based mutation, automatic TTL cleanup), and delivers via
    nodemailer if providers.email.{host,port,username,password} is set OR
    falls back to log.info('auth', 'email magic link issued', ...) for dev.
    Verify accepts the token, marks it used, creates the same DashCaddy
    session cookie that TOTP uses (single global cookie model).

createAuthProviderRegistry() composes both implementations and exposes
them via /api/v1/auth/login/{methods, :provider/initiate, :provider/verify,
recovery-info} and /api/v1/auth/disable/:provider. PUBLIC_ROUTES + CSRF
exemptions updated to use :provider placeholder (parameterized for future
providers).

Test fix: src/utilities/middleware.js PUBLIC_ROUTES and csrf-protection.js
both switched to the :provider form because the prior literal 'totp'
wouldn't match the parameterized mount path Express 4.22 produces.

Test fix: __tests__/public-routes-drift.test.js extractMountPath() was
broken for Express 4.22's new ^/path/?(?=/|$) source format (no \?\?
terminator in the literal-mount case). Rewrote the parser to normalize
escaped slashes + trailing lookaheads instead of relying on regex
matching against the raw source.

New: __tests__/auth-provider-registry.test.js — 9 tests covering registry
composition, getProvider round-trip, listEnabled no-secrets-leak guarantee,
enabled-flag respect, listAll vs listEnabled distinction, email provider
dev-console fallback (token written to JSON store + log.info with
deliveredVia: 'dev-console' + response masked), verify rejects unknown
tokens via AuthenticationError.

Tests: 1241/1241 passing across 46 suites (was 1232; +9 new).

DNS2 deploy: this commit + bump VERSION to 1.16.0 + publish tarball to
get.dashcaddy.net + docker build + bash start.sh. The image-layer migration
from DC-050 also runs on first container recreate post-merge.

CHANGELOG.md

@@ -8,6 +8,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 
 ### Added
+- **Pluggable `AuthProvider` framework + TOTP + EmailMagicLink providers (DC-046 + DC-047).** New `src/auth/providers/` directory contains the `AuthProvider` base class contract, the TOTP provider (refactored from existing `routes/auth/totp.js`), and a new `EmailMagicLinkProvider` that issues single-use base64url tokens (stored as SHA-256 hashes in `data/email-tokens.json`), sends via the existing nodemailer config (or logs to console + `log.info('email magic link issued')` in dev fallback). `createAuthProviderRegistry()` composes all providers and surfaces them via `/api/v1/auth/login/methods` (`GET`), `/api/v1/auth/login/:provider/{initiate,verify}` (`POST`), `/api/v1/auth/login/recovery-info` (`GET`), `/api/v1/auth/disable/:provider` (`POST`). Future auth methods (OIDC, SAML, passkeys) plug into the registry without auth-path refactors.
 - **`platform-paths.assertSafe()` — DC-046 hardening.** Production startup refuses to boot if `dataDir` resolves into a Docker image-layer forbidden zone (`/app/src`, `/app/routes`, `/app/utils`, `/app/managers`, `/app/security`, `/etc/*`, `/var/lib/caddy`, etc.). Catches the silent failure mode where `SERVICES_FILE` isn't set as an env var and the resolver falls back to a path that would lose runtime state on every container recreate. Bypassed with `SKIP_DATA_DIR_GUARD=1` for emergency legacy setups.
 - **`platform-paths.isMountedCheck(dir)`.** Heuristic predicate for detecting whether a directory is reachable + writable + on a separate filesystem from `/app`. Used by `start.sh` migration step to no-op safely on fresh installs.
 - **`start.sh` one-time image-layer migration step.** Runs before `docker run`. Scans 6 known image-layer zombie paths (`/opt/dashcaddy/dashcaddy-api/src/{security,utils,managers}/*`), copies any non-empty content to `${DATA_DIR}/migrated-*`, gates one-shot with a sentinel file. Idempotent. Recovers the 140KB `error.log` and any license-secret that landed in the image layer pre-DC-039.