feat: Phase 6 — push sources, upload path and Paper plugin

[TheMeinerLP]

Replaces #8, closed. Same content, rebuilt as a single squash commit on clean/phase-6-push because a secret scanner flagged disposable test credentials in the old commit history and history cannot be rewritten in this environment. See the closing comment on #8 for detail.

The last phase of the Apus spec. Stacked on PR #7.

Until now Apus pulled worlds — from an S3 bucket or a Pterodactyl panel. This adds the reverse: a running Paper server pushes its own world, and a user can upload one through the UI.

What this delivers

• paper-worldpush — a Paper plugin that copies a live world consistently (pause autosave, save once, copy, resume) and incrementally, uploading only region files whose mtime or checksum changed. All copying and uploading happens off the server thread; only the save itself touches the main thread, because Bukkit requires it.
• Push and upload connectors in the ingest module, plus the wiring so those source types actually run.
• POST /api/uploads with presigned multipart, so world files never travel through the API.
• POST /api/push/{token} — the one endpoint that authenticates with a tenant-bound service token rather than a user login, because a server plugin must not stop working when a person leaves the team.
• Push tokens are now provisioned by the operator: cryptographically random, one per tenant, written only into a Secret. Never in status, events or logs — status carries the Secret's name at most.

Upload restrictions, measured rather than claimed

A presigned URL is a transferable credential: what it permits, it permits to anyone holding it. So these were verified empirically against real MinIO rather than asserted.

• Prefix confinement is structural — the S3 key is a pure function of the server-derived tenant namespace, never client input.
• Redirecting a presigned part to a different key → 403 SignatureDoesNotMatch.
• Exceeding the declared part size → 403. This was the one expected to be weak; it is not.
• Total size cap is enforced at completion via ListParts, never presigned — an oversized upload is aborted and never becomes a readable object.

Caveat stated plainly: verified against MinIO, not independently against Ceph RGW. Same SigV4 mechanism, but that is an inference, not a second measurement.

An unresolved design conflict, stated rather than hidden

The plugin pushes many individual raw region files incrementally. The staged connector expects one object per version. The HTTP call between them is now correct, but an ingest triggered that way would likely find nothing usable.

The upload path (single archive) works end to end and is proven by PushIngestEndToEndTest against real MinIO. The plugin path needs its own design pass.

This surfaced because the plugin and the API endpoint were built simultaneously in separate worktrees — each side's assumption was reasonable, and neither could see the other's.

Also in this PR

• The design spec now opens with a "Stand der Umsetzung" section: which phases shipped, that sharding was deliberately not built after its spike, and what is knowingly open.
• Spec corrections: several modules still said Java 21 where everything is on 25, and several "open points" in section 15 had been resolved during implementation.
• Fixed a stale assertion in the ingest/render contract test. It listed expected bundle objects exactly and broke once level.dat was legitimately added by the writer. The check now requires every mandatory object and still rejects genuinely unexpected ones, without breaking on spec-documented sidecars.

Known gaps

• RBAC for the API's token lookup is broader than ideal (cluster-wide Secret read); a narrower path is documented but not implemented. No Helm or Kustomize manifests exist in the repo, so RBAC lives in javadoc only.
• The plugin's save window — pause autosave, save, copy, resume — is untested; that needs a real Paper server, which spec section 13.2 already concedes.
• The OIDC flow has still never run against a real broker; none is chosen (spec section 15).

[TheMeinerLP]

Superseded by #24. Rebuilt as a fresh squash on a base where the shell env-var validation already uses the require_env helper (and, for the phase-1 lineage, a docs correction to a plan file's stale code sample), so no follow-up fix commit is needed on top. Closing in favor of #24.