Add self-contained Docker Compose stacks for pivoine.art infrastructure

Migrated 11 services from monolithic docker-compose project into independent stacks,
each with dedicated databases, minimal .env configuration, and bind-mount data volumes.

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

README.md

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+# Stacks
+
+Self-contained Docker Compose stacks for `pivoine.art` infrastructure.
+
+Each stack is independently deployable with its own `compose.yml` and `.env`. All persistent data lives in `../.data/<stack>/`.
+
+## Stacks
+
+| Stack | Description | Services |
+|---|---|---|
+| `traefik` | Reverse proxy, TLS termination | traefik |
+| `mailpit` | SMTP relay (no web UI) | mailpit |
+| `watchtower` | Automatic container updates | watchtower |
+| `umami` | Web analytics | umami, db |
+| `immich` | Photo & video management | immich, ml, redis, db |
+| `joplin` | Note sync server | joplin, db |
+| `mattermost` | Team chat | mattermost, db |
+| `gitea` | Git hosting + CI runner | gitea, runner, db |
+| `coolify` | Deployment platform | coolify, realtime, redis, db |
+| `sexy` | pivoine.art website | directus, frontend, redis, db |
+| `vaultwarden` | Password manager | vaultwarden |
+
+## Deployment
+
+```bash
+# Sync a stack to VPS
+rsync -avz <stack>/ vps:~/stacks/<stack>/
+
+# Start a stack
+ssh vps 'cd ~/stacks/<stack> && docker compose up -d'
+```
+
+## Network
+
+All stacks share the external `falcon_network` Docker network for inter-service communication (e.g. traefik routing, mailpit SMTP).
+
+## Data
+
+Persistent data is stored in `~/stacks/.data/<stack>/` on the VPS using bind mounts. Database stacks use dedicated Postgres instances with simple credentials.