chore: prepare the 3.5.0 release

Turns the CHANGELOG's Unreleased section into `## 3.5.0` and bumps
`src.__version__`, the value plugin `ledmatrix_min_version` floors compare
against. No behaviour change; nothing outside the CHANGELOG, `src/__init__.py`
and one docs line is touched.

The staged entries are reshaped into the `### ` subsections every released
section already uses, and the "new modules a plugin may import via `src.*`"
block moves to the top as the plugin-facing summary, the same shape as 3.4.0.
Its floor, written as "the release that ships this" while it was staged, is now
3.5.0, and `docs/SPORTS_UNIFICATION.md` says 3.5.0 for `sports_helpers.py`
instead of "(unreleased)".

Four merged changes had never been written down. They are added under the
subsection each belongs to, from the commits and their measurements:

- the idle back-off clamped to the next kickoff (#599)
- concurrent ESPN date chunks (#596)
- the three web routes that consulted plugin manifests before anything had
  discovered plugins, one of which wrote a plugin API key to config.json in
  plain text (#594)
- the cache permission fix and its systemd unit changes (#593), which get
  their own subsection

No tag and no release: `scripts/check_release_version.py v3.5.0` passes, so
tagging is a separate, deliberate step.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Rqzd6Nz2bQJp5K7DD5dS4X

docs/SPORTS_UNIFICATION.md

@@ -81,7 +81,7 @@ src/common/
   sports_scroll.py       SportsScrollDisplay / …Manager — scroll orchestration
                          (content building stays in the plugins)
   sports_helpers.py      clamp/logo/rotation free functions + SportsHelpersMixin
-                         (unreleased) — the helpers byte-identical in the
+                         (3.5.0) — the helpers byte-identical in the
                          plugins' sports.py, and the _favorite_key seam
 ```