Durability pass: tests, clearer diversity/classify behavior, Calm Filters foundation

- Add pytest suite (34 tests) covering scoring thresholds, dedup clustering +
  representative selection + time window, brief source/category diversity,
  avoid-term phrase matching, and text canonicalization/truncation.
- Rewrite _select_diverse with an explicit, tested contract (best-first, one
  per source, backfill, then inject a second category by evicting the
  lowest-ranked pick).
- classify_articles now returns attempted/succeeded/skipped (ClassifyReport) so
  silent model failures are visible in both the cycle and classify output.
- Fix clean_text truncation to stay within max_len (ellipsis no longer
  overshoots).
- New filters.py: canonical FilterPrefs shape (include/mute topics+flavors,
  avoid_terms, pauses) and pure word/phrase-boundary matching engine seeding
  Calm Filters. Not yet wired into the API.

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

goodnews/briefs.py

@@ -141,21 +141,30 @@ def _candidate_articles(
 
 
 def _select_diverse(rows: list[sqlite3.Row], limit: int) -> list[sqlite3.Row]:
-    selected = []
-    seen_sources = set()
-    seen_categories = set()
+    """Pick up to `limit` items from `rows` (already ranked best-first).
 
+    Contract:
+    1. Prefer higher-ranked items.
+    2. Source diversity: take at most one item per source while other sources
+       remain; only repeat a source once distinct sources are exhausted.
+    3. Category diversity: if the result ended up single-category and a different
+       category is available in the pool, swap in the highest-ranked off-category
+       candidate by evicting the lowest-ranked currently-selected item (so we
+       gain breadth without dropping a higher-ranked pick).
+    """
+    selected: list[sqlite3.Row] = []
+    seen_sources: set = set()
+
+    # Pass 1: best-first, one per source.
     for row in rows:
         if len(selected) >= limit:
             break
-        source = row["source_name"]
-        category = row["default_category"]
-        if source in seen_sources and len(rows) > limit:
+        if row["source_name"] in seen_sources:
             continue
         selected.append(row)
-        seen_sources.add(source)
-        seen_categories.add(category)
+        seen_sources.add(row["source_name"])
 
+    # Pass 2: if short on distinct sources, backfill best-first regardless.
     if len(selected) < limit:
         selected_ids = {row["id"] for row in selected}
         for row in rows:
@@ -166,13 +175,15 @@ def _select_diverse(rows: list[sqlite3.Row], limit: int) -> list[sqlite3.Row]:
             selected.append(row)
             selected_ids.add(row["id"])
 
-    if len(seen_categories) < 2 and len(rows) > limit:
+    # Pass 3: ensure >= 2 categories when the pool allows it.
+    categories = {row["default_category"] for row in selected}
+    if len(categories) < 2:
         selected_ids = {row["id"] for row in selected}
         for row in rows:
             if row["id"] in selected_ids:
                 continue
-            if row["default_category"] not in seen_categories:
-                selected[-1] = row
+            if row["default_category"] not in categories:
+                selected[-1] = row  # evict the lowest-ranked selected item
                 break
 
     return selected