"Since you last visited" cue + PWA install (add to home screen)

Two calm returning-reader features.

Since-last-visit (Highlights companion, not a nav lane — per Codex):
* queries.feed gains a `since` filter; GET /api/since?ts= returns the count +
  a few accepted/non-dup/visible articles discovered since the reader's last
  visit (boundary-respecting; invalid/future ts → 0, no error).
* Home stores last_seen in localStorage (reads prev, then stamps now); on
  Highlights, a gentle "Since you were last here, N new calm reads came in"
  note with a "See what's new" reveal of a compact inline section. Dismissible.
  No badges, no unread counts, no "missed" language.

PWA:
* Real PNG icons (192/512 + full-bleed maskable) rasterized from favicon.svg;
  manifest fixed (azure theme to match the brand, PNG icons); apple-touch-icon.
* Minimal service worker: precache the app shell, always-fresh API + /a/ pages.
* Gentle, dismissible install banner (beforeinstallprompt → Install; iOS → the
  Share → Add to Home Screen hint). Never nags.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

goodnews/queries.py

@@ -64,6 +64,7 @@ def feed(
     sort: str = "ranked",
     follow_sources: list[int] | None = None,
     follow_tags: list[str] | None = None,
+    since: str | None = None,
 ) -> list[dict]:
     """Return articles with categorical filters applied in SQL.
 
@@ -116,6 +117,11 @@ def feed(
         clauses.append("a.source_id = ?")
         params.append(source_id)
 
+    if since:
+        # "New since last visit": articles discovered after the reader's last visit.
+        clauses.append("a.discovered_at > ?")
+        params.append(since)
+
     # "Following" feed: articles from a followed source OR carrying a followed tag.
     # Passing either list (even empty) switches to following mode; no follows → none.
     if follow_sources is not None or follow_tags is not None: