Add upgrade runbook + bake memory limit and LITELLM_MIGRATION_DIR into packaging

The 2026-10-02 outage had three causes the runbook now prevents:
- floating tag drift (README now mandates exact-tag pinning)
- 256MB default cgroup cap OOM-killing the prisma migration engine
  (manifest now sets memoryLimit: 2147483648)
- no _prisma_migrations ledger -> P3005 crash-loop on boot
  (start.sh now exports LITELLM_MIGRATION_DIR=/app/data/migrations)

README 'Updating LiteLLM' section rewritten from the old (wrong)
floating-tag procedure into a runbook with rules + verify checklist.

CloudronManifest.json

@@ -8,6 +8,7 @@
   "upstreamVersion": "1.84.0",
   "healthCheckPath": "/health/readiness",
   "httpPort": 4000,
+  "memoryLimit": 2147483648,
   "manifestVersion": 2,
   "website": "https://litellm.ai",
   "contactEmail": "botbot@hermes",