P5: OSS readiness — docs site, contributor onboarding, e2e harness

P5-01 — Documentation site under docs/book/ rendered with mdBook
(downloaded via Makefile, same static-binary pattern as Tailwind).
Structured chapters: getting started, concepts, operations,
security, reference. `make docs` / `make docs-watch`. Generated
output gitignored.

P5-02 — CONTRIBUTING.md rewritten from placeholder to a full
guide. CODE_OF_CONDUCT.md adapted from Contributor Covenant for a
single-maintainer project. .gitea/issue_template/{bug,feature}.md
and PULL_REQUEST_TEMPLATE.md.

P5-04 — Six README screenshots captured live from a fresh server
bootstrap (login, empty dashboard, add-host, alerts, settings,
audit log). README rewritten to centre the screenshot grid and
link out to the docs site.

P5-05 — SECURITY.md with disclosure policy (3-day ack, 30-day
default window), scope in/out, threat-model summary, operator
hardening checklist. Mirrored as a docs-site chapter.

P5-06 — End-to-end test harness. e2e/compose.e2e.yml brings up
server + sibling Linux agent (alpine + restic) + restic/rest-server.
Agent uses announce-and-approve so Playwright can drive the full
operator flow: bootstrap → login → accept pending → backup →
verify terminal status. Second spec scrapes /metrics to assert
the P6-04 endpoint surface. .gitea/workflows/e2e.yml runs on every
PR; local how-to in docs/e2e.md.

docs/book/src/contributing.md

@@ -0,0 +1,17 @@
+# Contributing
+
+Full contributor guide:
+[`CONTRIBUTING.md`](https://gitea.dcglab.co.uk/steve/restic-manager/src/branch/main/CONTRIBUTING.md)
+in the repository root.
+
+The short version:
+
+- Open an issue first for non-trivial changes; the design is
+  still moving and unsolicited large PRs may conflict with
+  in-flight work.
+- `make lint test` must pass.
+- One logical change per commit, no `Co-Authored-By` trailers.
+- UK English in identifiers and comments; comments explain the
+  **why** not the **what**.
+
+Code of conduct: [`CODE_OF_CONDUCT.md`](https://gitea.dcglab.co.uk/steve/restic-manager/src/branch/main/CODE_OF_CONDUCT.md).