docs: weekly digest feature design

Aggregates posts by ISO week into summary pages and an RSS feed.
Approved design — ready for implementation planning.

docs/plans/2026-02-25-weekly-digest-design.md

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+# Weekly Digest Feature Design
+
+**Date:** 2026-02-25
+**Status:** APPROVED
+
+## Overview
+
+A weekly digest that aggregates all posts from a given ISO week into a single summary page and RSS feed item. Subscribers get one update per week instead of per-post — a "slow RSS" feed.
+
+## Requirements
+
+- **Included post types:** All except replies (articles, notes, photos, bookmarks, likes, reposts)
+- **Format:** Summary list — title + link + short excerpt for each post, grouped by type
+- **Likes/bookmarks (no title):** Show target URL as label (e.g. "Liked: https://example.com/post")
+- **Week definition:** ISO 8601 weeks (Monday–Sunday)
+- **Empty weeks:** Skipped entirely (no page or feed item generated)
+- **Output:**
+  - HTML page per week at `/digest/YYYY/WNN/`
+  - Paginated index at `/digest/`
+  - RSS feed at `/digest/feed.xml`
+
+## Approach
+
+Eleventy collection + pagination (same pattern as `categoryFeeds`). Pure build-time, no plugins, no external data sources.
+
+## Design
+
+### 1. Collection: `weeklyDigests`
+
+Added in `eleventy.config.js`. Groups all published posts (excluding replies) by ISO week number.
+
+Each entry in the collection:
+
+```javascript
+{
+  year: 2026,
+  week: 9,
+  slug: "2026/W09",
+  label: "Week 9, 2026",
+  startDate: "2026-02-23",   // Monday
+  endDate: "2026-03-01",     // Sunday
+  posts: [ /* all posts, newest-first */ ],
+  byType: {
+    articles: [...],
+    notes: [...],
+    photos: [...],
+    bookmarks: [...],
+    likes: [...],
+    reposts: [...]
+  }
+}
+```
+
+- `byType` is pre-computed so templates don't filter
+- Post type detection reuses blog.njk logic (check likeOf, bookmarkOf, repostOf, inReplyTo, photo, title)
+- Empty types omitted from `byType`
+- Collection sorted newest-week-first
+
+### 2. Templates
+
+**`digest.njk`** — Individual digest page
+
+- Paginated over `weeklyDigests` collection
+- Permalink: `/digest/2026/W09/`
+- Layout: `base.njk` with sidebar
+- Heading: "Week 9, 2026 — Feb 23 – Mar 1"
+- Sections per type present that week:
+  - Articles/notes: title or content excerpt + date + permalink
+  - Photos: thumbnail + caption excerpt + permalink
+  - Bookmarks: "Bookmarked: https://target-url" + date + permalink
+  - Likes: "Liked: https://target-url" + date + permalink
+  - Reposts: "Reposted: https://target-url" + date + permalink
+- Previous/next digest navigation at bottom
+
+**`digest-index.njk`** — Paginated index
+
+- Permalink: `/digest/` (paginated: `/digest/page/2/`)
+- Lists: week label, date range, post count, link to digest page
+- 20 digests per page
+
+**`digest-feed.njk`** — RSS feed
+
+- Permalink: `/digest/feed.xml`
+- Each `<item>` = one week's digest
+- Title: "Week 9, 2026 (Feb 23 – Mar 1)"
+- Description: HTML summary list (grouped by type, same as HTML page)
+- pubDate: Sunday (end of week)
+- Latest 20 digests
+
+### 3. Discovery
+
+- `<link rel="alternate">` for digest feed in `base.njk`
+- "Digest" navigation item (conditional on digests existing)
+
+### 4. Files Changed
+
+| File | Change |
+|------|--------|
+| `eleventy.config.js` | Add `weeklyDigests` collection |
+| `digest.njk` | New — individual digest page |
+| `digest-index.njk` | New — paginated index |
+| `digest-feed.njk` | New — RSS feed |
+| `_includes/layouts/base.njk` | Add alternate link for digest feed |
+
+No new dependencies. No data files. No Indiekit plugin changes.