fix: Kuma import tokens/paused, Docker hardening, migrate-secrets idempotency

1. Kuma import now maps push monitor tokens (generates crypto/rand
   token) and paused state (Active=false → Paused=true). Previously
   push monitors imported with empty token sat DOWN forever, and
   paused Kuma monitors came in unpaused and started alerting.

2. Dockerfile adds HEALTHCHECK against /api/health on port 8080.
   Container orchestrators can now detect unhealthy instances.

3. migrate-secrets sets the encryptor before loading alerts, so
   already-encrypted settings are decrypted correctly on second run
   instead of failing with a JSON unmarshal error.

4. docker-compose.yml adds container hardening: read_only filesystem,
   cap_drop ALL, no-new-privileges, tmpfs for /tmp.

deploy/docker-compose.yml

@@ -5,6 +5,13 @@ services:
       dockerfile: Dockerfile
     container_name: uptop
     restart: unless-stopped
+    read_only: true
+    cap_drop:
+      - ALL
+    security_opt:
+      - no-new-privileges:true
+    tmpfs:
+      - /tmp
     ports:
       - "23234:23234"
       - "8080:8080"