docs: add the governance and CI-operations files the project never had

The repo had no SECURITY.md, CONTRIBUTING.md, code of conduct, or issue
and PR templates. For a client that handles Jellyfin credentials and
ships signed binaries, the missing one that actually matters is
SECURITY.md: there was no stated way to report a vulnerability
privately, so the only available channel was the public tracker.

CONTRIBUTING.md documents the gates as they now stand, including the
three ratchets and which direction each is allowed to move, and the two
rules that surprise people: bug fixes start with a failing test, and
Jellyfin's taxonomy stays in Rust.

The bug template asks the three playback questions -- streaming or
downloaded, transcoding or direct, music or video -- because those
answers decide which of several very different code paths a report is
about, and reconstructing them over several round trips is most of the
cost of a playback bug report.

docs/build/ci-operations.md is the missing operations manual: how to
change the builder image and in what order (image pushed before the
workflow that names it, or CI breaks), why tags are dated rather than
:latest or per-SHA, what each secret is for, and what losing the updater
private key would mean -- installed desktop clients only accept payloads
signed by the key matching the public key they shipped with, so losing it
means everyone reinstalls by hand.

Disk exhaustion on the runner is documented as a manual check rather than
a scheduled job. A daily job would occupy the only slot on a single-slot
runner and pull the whole builder image to run `df` -- and `df` inside a
container does not reliably describe the host's disk, so it would spend
real build capacity reporting a number that might be wrong. What the doc
records instead is the part that is actually hard to rediscover: the
symptoms (cargo dying mid-link, docker refusing to pull, actions/cache
quietly not saving) and that `docker volume prune` needs `-a` to touch
named volumes, which is how it filled up unnoticed.

Two things in these docs are stated plainly because they are true and
were not written down anywhere: without branch protection every gate in
the pipeline is advisory, and the Gitea instance -- canonical remote,
signing secrets, registry, runner -- is not backed up by anything in this
repository.

.gitea/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,22 @@
+## What and why
+
+<!-- What changes, and the reason. The diff shows the what; the why is what
+     the commit log is for. -->
+
+## How it was verified
+
+<!-- What you actually ran or clicked. "Tests pass" on its own says little;
+     "played a transcoded episode on Android, seeked twice, backgrounded it"
+     says a lot. -->
+
+## Checklist
+
+- [ ] `bun run check`, `bun run test`, `bun run format:check`, `bun run lint`
+- [ ] `cargo fmt`, `cargo clippy --all-targets -- -D warnings`, `cargo test`
+- [ ] `bun run check:boundary` — no Jellyfin taxonomy in the frontend
+- [ ] New requirement-implementing code carries a `TRACES:` comment, and every
+      ID it names exists in `docs/requirements.md` (`bun run traces:validate`)
+- [ ] **Bug fix:** a test that reproduces it was written *first* and observed
+      failing before the fix
+- [ ] Android source edits were made in `src-tauri/android/src` and synced with
+      `scripts/sync-android-sources.sh` (never edit `gen/` directly)