docs(vibe): add applications/ guidebook (webapp + url-shortener)

Tree-docs guidebook under vibe/guidebooks/applications/ documenting the common
app pattern and two contrasting archetypes, drilling into lab-ecosystem/01-factory
(bidirectional):

- README.md  : the shared app pattern (repo = Dockerfile + chart + optional iac +
  CI; ArgoCD app-of-apps; the <app> join key; .fr vs .lab ingress conventions) +
  a two-archetype comparison.
- webapp.md  : canonical Go + Postgres exemplar (chart, VaultAuth/Static/Dynamic
  CRDs, inline iac vs the shared app_roles module, CI); notes the current nuance
  that the live pod still uses the static pgbouncer_auth DATABASE_URL.
- url-shortener.md : Rust + SQLite-on-Longhorn-RWO counterpart (single replica,
  no iac/no Vault, CI mirrors the upstream image); the power-cut recovery story.

erp is referenced in prose only (its own guidebook lands next). Sibling-repo code
via full gitea URLs; 2 mermaid diagrams MCP-validated; zero dead links.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

vibe/guidebooks/README.md

@@ -38,6 +38,7 @@ flowchart LR
 | [Factory provisioning](factory-provisioning/README.md) | Deep dive into how factory provisions everything: Ansible playbooks + roles and OpenTofu | ✅ Active |
 | [Tools](tools/README.md) | Deep dive into the lab platform services in the `tools` namespace (Vault+VSO, Prometheus, Grafana, CrowdSec, poolers, Redis, Plausible, ClickHouse) | ✅ Active |
 | [CMS](cms/README.md) | Deep dive into the public Nuxt site arcodange.fr + its Cloudflare DNS/tunnel/Turnstile and Zoho email IaC | ✅ Active |
+| [Applications](applications/README.md) | The deployed apps and the common pattern they share — webapp (Go + Postgres) and url-shortener (Rust + SQLite); erp has its own guidebook | ✅ Active |
 
 ## Rules to contribute