docs: re-home the project at tarnover/send and feature ffsend

[jaschadub]

Summary

Rewrites README.md and updates the docs to:

1. Present this repository (tarnover/send) as the home of the maintained fork, with the full lineage acknowledged: `mozilla/send → timvisee/send → tarnover/send`.
2. Surface `ffsend` prominently as a first-class client. It's now in the Quick start, the Clients section, the FAQ, and the security docs — explicitly recommended for any sensitive workflow because it sidesteps the operator-shipped-JS class of risk.
3. Cut over deployment instructions to GHCR + tarnover URLs while keeping a cross-reference to the upstream `timvisee/send` GitLab image (the two stay protocol-compatible).
4. Add `docs/security.md` — threat model, the specific hardening this fork carries vs upstream (with file:line citations), CVE posture, and known limitations.

Files touched

• `README.md` — full rewrite: new badges (GHA + GHCR), Quick-start with `ghcr.io/tarnover/send`, "What's different in this fork" section, expanded Clients table, Acknowledgements, contributing pointer that asks contributors to also upstream where relevant.
• `docs/security.md` — new file. Documents threat model, per-finding rationale for the hardening landed in #1, CVE posture (linked to #3), and known limitations.
• `docs/docker.md` — default image is now `ghcr.io/tarnover/send:latest`. Adds a tag-list table. Cross-references the upstream GitLab image. `timviseesend` example network name renamed to `sendnet`.
• `docs/deployment.md` — git clone URL points at tarnover.
• `docs/AWS.md` — git clone URL + systemd `Documentation=` URL point at tarnover.
• `docs/faq.md` — drops the stale `send.firefox.com/legal` telemetry pointer (this fork ships no telemetry), updates source link, adds a "Is there a command-line client?" question recommending `ffsend`.

What I deliberately didn't change

• Upstream cross-references for the GitLab image and for `send-docker-compose` (the upstream still hosts those — there is no point shadowing them here).
• The MPL license / Mozilla copyright in headers and `LICENSE` — that's the legal lineage and stays.
• Anything outside docs and the README. Code is unchanged.

Test plan

• Visual check on rendered README at https://github.com/tarnover/send/tree/docs/upstream-and-clients.
• All in-repo links resolve (`README.md` and `docs/security.md` reference `docs/docker.md`, `docs/deployment.md`, `docs/encryption.md`, etc.).
• No remaining `gitlab.com/timvisee/send` URLs that should have been replaced (a couple are intentionally kept as cross-references to upstream).