Add backlog note: feed health (recency/frequency, dead-feed detection)

Captured while migrating 108 feeds from Inoreader, which surfaced ~19
feeds manually flagged dead over there with no equivalent detection in
this app yet.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EZ7ikw2ukGJFTHE3bJS7zL

1 files changed, 36 insertions, 0 deletions

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+# Backlog
+
+## Feed health: recency/frequency tracking + dead-feed detection
+
+**Motivation:** migrating 108 feeds from Inoreader surfaced ~19 feeds the
+user had manually flagged "dead?" over there — there's currently no way
+for this app to tell that on its own. Want automatic detection instead of
+relying on manual tagging carried over from another tool.
+
+**What's needed:**
+- Track each feed's normal posting cadence (e.g. median interval between
+  the last N articles' `publishedAt` — already stored on `ArticleEntity`,
+  no new fetch required, just a read-side computation).
+- Surface a "stale" indicator on the feed list when a feed has gone
+  quiet for meaningfully longer than *its own* historical cadence — not
+  a fixed global threshold. A feed that normally posts yearly shouldn't
+  be flagged after 3 quiet months; a feed that normally posts daily
+  should be flagged after a couple of quiet weeks.
+- Treat "feed URL itself is broken" as a distinct, stronger signal from
+  "author's just gone quiet": `FeedEntity.lastError` already records
+  fetch failures (404s, parked domains, etc.) — a feed with several
+  consecutive failures is a different case than a live, fetchable feed
+  that simply has no new posts.
+
+**Rough shape (not yet built):**
+- Compute per-feed on read (no new table needed at first): take the last
+  N `publishedAt` values for a feed, find the median gap, compare against
+  time since the most recent post. Flag if the current gap is some
+  multiple (e.g. 3x) of the historical median.
+- `FeedListScreen`: a small badge next to the feed row when flagged,
+  distinguishing "quiet" (posting-cadence signal) from "broken"
+  (`lastError` signal) since they call for different user action (wait
+  it out vs. go look for a replacement feed).
+- Out of scope for now: any automated "find where the author moved"
+  logic — that's a manual research task when a feed's actually declared
+  dead, not something to build detection for.