[uptime-kuma] Codify MySQL monitor (id=663) via idempotent sync CronJob

## Context

Monitor id 663 "MySQL Standalone (dbaas)" was created manually yesterday via
the `uptime-kuma-api` Python library when the dbaas stack migrated from
InnoDB Cluster to standalone MySQL. It worked and was UP, but lived only in
Uptime Kuma's MariaDB — if UK's DB were wiped or restored from an older
backup, the monitor would be lost.

## This change

Adds declarative, self-healing management for internal-service monitors
(databases, non-HTTP endpoints) that can't be discovered from ingress
annotations. Modelled on the existing `external-monitor-sync` CronJob.

- `local.internal_monitors` — list of desired monitors (name, type,
  connection string, Vault password key, interval, retries). Seeded with
  the MySQL Standalone monitor. Add new entries here to manage more.
- `kubernetes_secret.internal_monitor_sync` — pulls admin password and all
  referenced DB passwords from Vault `secret/viktor` at apply time. Secret
  key names are derived from monitor name (`DB_PASSWORD_<upper_snake>`).
- `kubernetes_config_map_v1.internal_monitor_targets` — renders the target
  list to JSON for the sync container.
- `kubernetes_cron_job_v1.internal_monitor_sync` — runs every 10 min,
  looks up monitors by name, creates if missing, patches if drifted,
  leaves id and history untouched when already in desired state.

## Why this approach (Option B, not a Terraform provider)

The `louislam/uptime-kuma` Terraform provider does NOT exist in the public
registry (verified — only a CLI tool of the same name). Option A from the
task brief was therefore unavailable. Option B (idempotent K8s CronJob)
matches the established pattern in the same module for
`external-monitor-sync` — no new machinery introduced.

## Monitor 663: no-op on first sync

Manual import was not possible (no provider → no state to import). The
sync job correctly identifies the existing monitor by name and reports:

  Monitor MySQL Standalone (dbaas) (id=663) already in desired state
  Internal monitor sync complete

DB heartbeats confirm monitor 663 stayed UP throughout with `status=1` and
`Rows: 1` responses every 60s — no disruption.

## Vault key — left manual (by design)

`secret/viktor` is not Terraform-managed anywhere in the repo (only read
via `data "vault_kv_secret_v2"`). It is a user-edited Vault entry holding
135 keys. The `uptimekuma_db_password` key was added manually yesterday;
this change does NOT codify it. Codifying the whole `secret/viktor` entry
is out of scope for this task (would need a separate migration + rotation
story). The sync job reads the existing value at apply time — so if the
value is ever rotated in Vault, the next sync picks it up.

## Plan + apply

  Plan: 3 to add, 0 to change, 0 to destroy.
  Apply complete! Resources: 3 added, 0 changed, 0 destroyed.
  Re-plan: No changes. Your infrastructure matches the configuration.

Also updated `.claude/skills/uptime-kuma/SKILL.md` with the new pattern.

Closes: code-ed2

.claude/skills/uptime-kuma/SKILL.md

@@ -155,3 +155,19 @@ Common port is 80. Exceptions:
 3. Add `time.sleep(0.3)` between bulk operations to avoid overloading
 4. Homepage dashboard widget slug: `cluster-internal`
 5. Cloudflare-proxied at `uptime.viktorbarzin.me`
+
+## Terraform-Managed Monitors
+
+There is NO `louislam/uptime-kuma` Terraform provider. Two patterns exist for
+declarative monitor management in this stack:
+
+- **External HTTPS monitors** — auto-discovered from ingress annotations by the
+  `external-monitor-sync` CronJob (`*/10 * * * *`). Opt-out via
+  `uptime.viktorbarzin.me/external-monitor: "false"` on the ingress.
+- **Internal monitors (DBs, non-HTTP)** — declared in the
+  `local.internal_monitors` list in `stacks/uptime-kuma/modules/uptime-kuma/main.tf`
+  and synced by the `internal-monitor-sync` CronJob. To add one, append to the
+  list (provide `name`, `type`, `database_connection_string`,
+  `database_password_vault_key`, `interval`, `retry_interval`, `max_retries`)
+  and `scripts/tg apply`. The sync is idempotent — looks up by name, creates
+  if missing, patches if drifted. Existing monitors keep their id and history.