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/lab-ecosystem/01-factory.md

@@ -52,6 +52,9 @@ The tested power-cut recovery sequence (Longhorn restore → Vault unseal → VS
 
 A Helm chart whose `templates/apps.yaml` loops over `values.gitea_applications` and emits one `Application` CRD per app. Each Application derives everything from the app name: `repoURL = https://gitea.arcodange.lab/<org>/<app>`, `path = chart`, `namespace = <app>` (`CreateNamespace=true`), with `syncPolicy.automated` `prune: true` + `selfHeal: true` by default.
 
+> [!TIP]
+> **Deeper dive:** the [Applications guidebook](../applications/README.md) maps what these `Application` CRDs deploy — the common app-repo pattern (Dockerfile + `chart/` + optional `iac/` + CI) every app in the list below shares, and the two archetypes (Go + Postgres vs Rust + SQLite).
+
 | App | Org override | Image Updater |
 |---|---|---|
 | `url-shortener` | — | — |
@@ -113,6 +116,7 @@ flowchart LR
 ## Cross-references
 
 - [Lab ecosystem hub](README.md) — the whole-lab map this page sits under.
+- [Applications guidebook](../applications/README.md) — the apps ArgoCD's app-of-apps deploys: the common app-repo pattern and the Go+Postgres / Rust+SQLite archetypes.
 - [02 · tools](02-tools.md) — what ArgoCD deploys into the `tools` namespace (incl. pgbouncer that consumes the PG `user_lookup()`).
 - [03 · cms](03-cms.md) — the CMS edge that `iac/cloudflare.tf` and `iac/ovh.tf` wire up.
 - [naming-conventions.md](naming-conventions.md) — the `<app>` join key these pillars share.