WordPress-style pages with home chooser

#27 in Riparion/riparion-cms — merged 2026-06-01

Adds WordPress-style hierarchical pages with a multi-region layout editor, plus a home chooser so / can render either the chronological feed or a static page (with /blog always serving the feed).

Commits

• Add WordPress-style pages with home chooser — pages model, materialized-path tree, multi-region ScreenLayout editor, public reader, and the home_kind chooser.
• Move home-page source toggle from Appearance to Settings — relocates the feed-vs-page toggle to its more natural home.
• Fix pages/home-chooser bugs from review (PH1–PL6) — the review-driven fixes below.

Review fixes (PH1–PL6)

High

• PH1 — add the bare Axum route segments (rss, feed, atom, sitemap, uploads, site) to RESERVED_SLUGS so a page slug can't be silently shadowed by the Axum layer.
• PH2 — retry create_page with an incrementing slug suffix on a UNIQUE collision instead of surfacing a raw 500, mirroring the post-side helper.

Medium

• PM1 — #[serde(skip)] on RenderedSection.body_html; the reader renders from body_segments, so the pre-rendered HTML no longer crosses the wire.
• PM2 — show an editor-only banner when the configured home page is a draft, explaining the public sees the feed at / until it's published.

Low

• PL1 — correct the DFS comment and add a visited-set cycle guard in update_page_db.
• PL2 — extract mdx::render_segments, shared by the editor preview and the reader.
• PL3 — hoist validate_status/validate_featured_image to server/admin/mod.rs.
• PL4 — extract a snapshot_current callback in PageEditorForm.
• PL5 — make LayoutSelector::status_msg Option<Signal<String>>; drop the dead layout_msg signal.
• PL6 — add get_page_by_id_db + a shared assemble_page_detail to drop the id→path round trip on the home-page hot path.

Notes

• PH2's slug retry is scoped to create (matching posts); on update a duplicate slug still returns a generic error rather than silently renaming a slug the user typed.
• PM2's banner is in addition to PageView's existing generic "Draft preview" alert — it adds the home-chooser-specific context.

Verification

• cargo fmt --check — clean
• cargo clippy -D warnings on server (sqlite + postgres) and wasm — clean
• cargo test — 26 pass

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