docs: MCP gateway git/gitlab/paper-search; CT 109 plan; new backrest/db/arcane docs

- mcp-gateway.md: 26 servers (added git, gitlab, paper-search catalog bridge);
  document --pass-environment requirement for env var passthrough in DinD bridges
- ct-inventory.md + homelab-architecture.md: CT 109 IP conflict, CT 113 live
- proxmox-state.md §16: Loki, sshwifty, Alloy, IP conflict note, sidecar table
  expanded to CT 103/111/113; build order updated
- infra/portmap.md: CT 113 db section; QNAP TS-251D (Klipper/Mainsail)
- services/backrest.md: full backup strategy doc (new)
- services/databases.md: CT 113 postgres/WAL-G/pgAdmin doc (new)
- services/arcane.md: Docker management IDE doc (new)
- CHANGELOG.md: 2026-05-22 entry

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

services/homelab-architecture.md

@@ -25,7 +25,7 @@ across ~23 compose stacks. Principle: **self-host everything**, OIDC SSO,
   - `110` lxc **id** — Pocket-ID OIDC IdP (1c/1G/4G) → `.5`; migrated off 104 on 2026-05-20
   - `111` lxc **dev** — Coder + Gitea (12c/32G/60G) → `.42`; new 2026-05-20
   - `109` lxc **observe** *(planned)* — Prometheus/Grafana/Arcane/Dozzle/Homarr/Tinyauth; see §16 of `infra/proxmox-state.md`
-  - `113` lxc **db** *(planned)* — shared Postgres + pgbouncer once second NVMe lands
+  - `113` lxc **db** — postgres 17 + pgAdmin + WAL-G → Garage S3 + Arcane edge agent → `.6`; provisioned 2026-05-21
   - Storages: `local` (dir), `local-zfs` (zfspool, ~1.9T), `unas` (nfs, ~20T).
 - **Ubiquiti UniFi** — UDM "Home" (UDMA6A8, UCG Fiber) gateway + 4× U7-series
   APs. Controller at **`https://192.168.1.1`** (UniFi OS 5.0.16, SSO + MFA