chore: Add deploy plumbing for the share service

Add a hardened systemd unit, a Caddy reverse-proxy snippet that maps
/fabula/api/* to the loopback service, and Justfile build-server/
deploy-server recipes that build a static binary and ship + restart it.
Includes server/README documenting the API, config, and deploy steps.

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

server/README.md

@@ -0,0 +1,51 @@
+# share-svc
+
+Tiny Go + SQLite service that stores character-sheet JSON blobs and returns
+short, content-addressed IDs, so sheets share as `...?s=<id>` instead of a giant
+inline `?c=` payload.
+
+## API
+
+| Method | Path         | Body        | Response            |
+| ------ | ------------ | ----------- | ------------------- |
+| POST   | `/api/s`     | sheet JSON  | `{"id":"<id>"}`     |
+| GET    | `/api/s/{id}`| –           | sheet JSON          |
+
+IDs are `base64url(sha256(json))[:12]`, so identical sheets dedupe and re-sharing
+is idempotent. Payloads are stored deflated in a single SQLite file.
+
+### Config (env)
+
+| Var           | Default       | Notes                                            |
+| ------------- | ------------- | ------------------------------------------------ |
+| `ADDR`        | `:8090`       | Listen address. The unit binds `127.0.0.1:8090`. |
+| `DB_PATH`     | `shares.db`   | SQLite file path.                                |
+| `CORS_ORIGIN` | _(unset)_     | Set to allow a cross-origin dev frontend.        |
+
+## Deploy
+
+From the repo root:
+
+```sh
+just deploy-server                 # build (linux/amd64) + ship + restart
+just go-arch=arm64 deploy-server   # if the host is arm64
+```
+
+That installs `/usr/local/bin/share-svc` and `share-svc.service`, then
+`daemon-reload` + `enable --now` + `restart`. The SQLite DB lives in
+`/var/lib/share-svc/` (created by the unit's `StateDirectory`).
+
+### One-time Caddy setup
+
+Add the `/fabula/api/*` reverse-proxy block from `Caddyfile.snippet` to your site
+block (before the static `/fabula/*` handler) and reload Caddy.
+
+## Local dev
+
+`webpack serve` (port 8080) does not serve `/api/*`, so `?s=` sharing falls back
+to inline `?c=` links. To exercise short links locally, run the service and point
+the frontend at it:
+
+```sh
+CORS_ORIGIN=http://localhost:8080 go run ./server
+```