feat: add Database-Gateway Cloudron package (Infrastructure) [#639]

Database Gateway 0.24.0 as the 12th package: multi-stage Go build
(CGO required by the libpg_query parser, hence an alpine:3.23 runtime
matching upstream), native OIDC wired to the Cloudron platform identity
provider, postgresql addon storage with goose migrations applied at
start, jq-generated config + OPA policy seeded to /app/data. Verified
end-to-end against a throwaway postgres (migrations, policy compile,
startup to the OIDC handoff). Docs gardened to 12 packages.

Ticket: https://projects.knownelement.com/issues/639

Package-Workspace/Infrastructure/database-gateway/CHANGELOG.md

@@ -0,0 +1,18 @@
+# Changelog
+
+## [0.24.0] - 2026-09-01
+
+Initial Cloudron package for
+[Database Gateway](https://github.com/kazhuravlev/database-gateway) v0.24.0
+(ticket [#639](https://projects.knownelement.com/issues/639)).
+
+- Multi-stage Go build (golang:1.26-alpine → cloudron/base:3.2.0), frontend
+  embedded by upstream via `go:embed`.
+- Native OIDC login wired to the Cloudron platform identity provider
+  (`CLOUDRON_OIDC_ISSUER` / `CLIENT_ID` / `CLIENT_SECRET`); roles mapped from
+  the `groups` claim (`admins` → admin, `users` → user).
+- `postgresql` addon (16) for the gateway's own storage; migrations applied
+  on every start via `migrate-up`.
+- `/app/data/config.json` + `/app/data/opa/simple.rego` seeded on first run;
+  operators add database targets and OPA policy rules via the file manager.
+- Single HTTP listener on port 8080 (`httpPort`).