feat: add OIDC/SSO authentication (#3)

Add OpenID Connect (OIDC) authentication alongside existing
username/password login. Users can log in via any standards-compliant
OIDC provider (Keycloak, Authentik, Authelia, Google, Azure AD, Okta)
while preserving full backward compatibility.

- OIDC Fastify plugin with lazy discovery, PKCE, cookie-based sessions
- Login page OIDC button, auth hook updates, settings dialog badges
- 28 integration tests, OIDC setup guide with provider examples
- Fix pre-existing test failures (content-aware-crop, watermark, SVGZ)
- WAL checkpoint fix for SQLite test stability

Closes #3

# Conflicts:
#	apps/api/src/lib/env.ts
#	apps/api/src/routes/tools/watermark-image.ts
#	pnpm-lock.yaml
#	tests/integration/color-palette.test.ts
#	tests/integration/compare.test.ts
#	tests/integration/watermark-image.test.ts

docker/Dockerfile

@@ -273,7 +273,10 @@ ENV PORT=1349 \
     SESSION_DURATION_HOURS=168 \
     LOGIN_ATTEMPT_LIMIT=500 \
     LOG_LEVEL=info \
-    TRUST_PROXY=true
+    TRUST_PROXY=true \
+    OIDC_ENABLED=false \
+    EXTERNAL_URL= \
+    COOKIE_SECRET=
 
 # NVIDIA Container Toolkit env vars (harmless on non-GPU systems)
 ENV NVIDIA_VISIBLE_DEVICES=all \