contextator MCP server

### Added

- **A major image tag** (`1`, `1-slim`) follows the newest release of that major, for installations that take every
  `1.x` update under the compatibility policy (ADR-0137), whose announced exceptions still apply — the deprecated
  legacy HTTP+SSE transport and, for an installation run outside the container, the Node.js base. It is
  published from `1.0.0` on; `0.x` releases get none, since a `0.y.z` minor may still change a public contract.
- **The MCP Registry entry is published by the release workflow.** A final-release tag now publishes `server.json`
  from a `registry` job after the images, signing in with the run's GitHub OIDC identity instead of a maintainer's
  personal token. The job stops when `server.json`'s `version` does not match the tag; a pre-release tag publishes
  nothing to the registry.
- **1.0.0 fixtures for the compatibility promise** (ADR-0137). `test/integration/fixtures/schema-1.0.sql` is the database
  a 1.0.0 start leaves behind, with a few rows; the integration suite loads it, runs the bootstrap and requires the
  schema a fresh database gets. `test/contracts/fixtures/` gains the backup and export manifests 1.0.0 writes, beside
  the ones 0.7 wrote, which stay. Tests only; nothing at run time changes.

### Changed

- **The legacy HTTP+SSE transport now announces its removal date** (ADR-0139). Every legacy SSE response — the
  stream and its `/messages` channel, refusals included — carries `Deprecation: @1790899200` (RFC 9745, announced
  2026-10-02), `Sunset: Sat, 02 Oct 2027 00:00:00 GMT` (RFC 8594) and `Link` with `rel="deprecation"` and
  `rel="sunset"` pointing at `docs/compatibility.md`. `Deprecation` was `true` before. `sunset` and `link` are
  added to the CORS exposed headers, and the dashboard notice for a project with a recent legacy client now
  names the date and links the page instead of saying no date is set. Behaviour is unchanged: SSE keeps working
  until it is removed, and Streamable HTTP responses carry none of these headers.
- **Node.js 24 is now required** (ADR-0138). Both images (`full` and `slim`) are built on `node:24-bookworm-slim`,
  CI runs 24, and `engines.node` is `>=24`. Nothing changes for anyone running the image; an operator running the
  application outside the container on Node 22 has to move to 24.
- **`unpdf` 2** reads PDFs. Text extraction, the page-count cap and the title taken from the document's metadata
  work as before; the new major needs Node.js 24, which is why it follows the runtime move.
- **Two variables are classified before the contracts freeze** (ADR-0140). `CONTEXTATOR_EMBEDDED_PGDATA`, which the
  image's entrypoint hands to the application, is internal: it is not part of the supported environment, and the
  contract snapshot records it in a separate `internal` block. `BACKUP_STAGING_DIR`, which the README tells operators
  to set for the scheduled-backup sidecar and the chart sets, is a supported setting. The snapshot also records the
  cross-key startup refusals (for example `SECRET_KEY_PREVIOUS` without `SECRET_KEY`); they refuse what they refused.
- The CI changelog gate now also watches `test/contracts/fixtures/`: a pull request that changes an archive fixture
  must add a line under `[Unreleased]`, as one that changes a contract snapshot does.

### Deprecated

- `OPENAI_BASE_URL` (ADR-0140). It still sets where `EMBEDDING_PROVIDER=openai` sends requests when
  `EMBEDDING_BASE_URL` is empty, and nothing about that changes, but the server now logs one warning at startup while
  that is what it does. Set `EMBEDDING_BASE_URL` instead; moving changes the embedding model id once, so each project
  is re-indexed (see the notes beside `EMBEDDING_BASE_URL` in `.env.example`). It is removed no sooner than `2.0`.

### Removed

- **BREAKING: `CONFLUENCE_ALLOWED_HOSTS`** (deprecated in `0.6.0`; ADR-0140, ADR-0126 decision 7). A non-empty value
  now stops the server from starting, with a message that names `SOURCE_ALLOWED_HOSTS`; an empty or