feat(services): env doctor — surface empty vars, bad refs, unreadable secrets

GET /services/:id/env-check (read scope) reports, without exposing values:
- unresolved: ${{service.KEY}} refs that don't match a sibling service/env
- empty: variables with blank values
- unreadable: secrets whose ciphertext fails decryption (key rotation)

install.sh now generates SECRETS_KEY explicitly (was falling back to
JWT_SECRET — a rotation would orphan ciphertext). .env.example documents it.

Surfaces: CLI `services env-check`, MCP containr_services_env_check
(90 tools), OpenAPI path, and a warning banner in the service Variables
panel listing each issue.

Verified live: injected a bogus ${{nosuchservice.PORT}} var plus an
unreadable ciphertext row; the endpoint flagged both, ok=false.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

.env.example

@@ -39,6 +39,10 @@ REDIS_URL=redis://:your_secure_redis_password@localhost:6379/0
 # Application Configuration
 # In production this must be a strong value with at least 32 characters.
 JWT_SECRET=SuperSecretJWTKey123456789!@#$%^&*()
+# Encrypts secret env vars at rest. Defaults to JWT_SECRET when unset;
+# setting it explicitly survives a JWT_SECRET rotation.
+SECRETS_KEY=
+
 # Shared secret for node-agent registration/heartbeat auth.
 CONTAINR_AGENT_AUTH_TOKEN=replace_with_strong_agent_secret
 # Optional rotation list (comma-separated). When set, this takes precedence.