Phase B1: multi-tag groupings model (backend)

Three-layer organization: primary topic (one per article, for ranking and
brief balance) + grouping tags (1-4 per article from a controlled vocabulary,
the organic "wandering" axis) + tonal flavor.

- taxonomy: add technology + learning topics; 4 calm tag families
  (Discovery & Wonder, People & Kindness, Solutions & Progress, Mind & Craft)
  defined in code, not the DB; ALLOWED_TAGS union + coerce_tags validation.
- db: article_tags(article_id, tag) join table + tag index.
- llm: tags added to the classifier json_schema (enum-constrained, maxItems 4)
  and system prompt; normalize_scores coerces tags; upsert_article_score
  replaces a row's tags atomically on every (re)classification.
- queries: feed gains a tag filter and exposes tags via group_concat; tag_counts.
- api: Article.tags, feed tag param, and /api/families with per-tag counts.
- tests: coerce/normalize/upsert/tag-filter/reclassify-replace/tag_counts +
  /api/families. 99 passing.

Corpus reclassify (re-tag + new primary topics) runs separately against the
local LLM. Frontend (B2) pairs with this; the live site is unchanged until then.

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

goodnews/db.py

@@ -70,6 +70,14 @@ CREATE TABLE IF NOT EXISTS article_scores (
     scored_at TEXT NOT NULL DEFAULT CURRENT_TIMESTAMP
 );
 
+CREATE TABLE IF NOT EXISTS article_tags (
+    article_id INTEGER NOT NULL REFERENCES articles(id) ON DELETE CASCADE,
+    tag TEXT NOT NULL,
+    PRIMARY KEY (article_id, tag)
+);
+
+CREATE INDEX IF NOT EXISTS idx_article_tags_tag ON article_tags(tag);
+
 CREATE TABLE IF NOT EXISTS article_embeddings (
     article_id INTEGER PRIMARY KEY REFERENCES articles(id) ON DELETE CASCADE,
     vector BLOB NOT NULL,