Follow source/topic — account-backed personalization (v1)

Per Codex — turn accounts into a real reason to return, without an algorithmic
feed. Durable interests (sources + tags), not moods.

* DB: user_follows (user_id, kind source|tag, value, unique).
* queries.feed gains follow_sources/follow_tags → the Following feed is
  "articles from a followed source OR carrying a followed tag", still respecting
  calm filters/boundaries.
* API: GET/POST/DELETE /api/follows (sign-in required; source ids validated);
  /api/feed?following=true resolves the user's follows (anon → empty, not error).
* Frontend: follows store (followKeys + toggleFollow, mirrors savedIds); a
  Follow button on source + tag/topic views; a "Following" lane in the nav with
  a tailored empty state; a Following management section in Account (unfollow).

Digest "From what you follow" deferred to v2 (brief stays first).

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

goodnews/db.py

@@ -257,6 +257,15 @@ CREATE TABLE IF NOT EXISTS feedback_replies (
 );
 CREATE INDEX IF NOT EXISTS idx_feedback_replies_fid ON feedback_replies(feedback_id);
 
+CREATE TABLE IF NOT EXISTS user_follows (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    user_id INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    kind TEXT NOT NULL,        -- 'source' | 'tag'
+    value TEXT NOT NULL,       -- source id (as text) or tag key
+    created_at TEXT NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    UNIQUE (user_id, kind, value)
+);
+
 CREATE TABLE IF NOT EXISTS digest_sends (
     id INTEGER PRIMARY KEY AUTOINCREMENT,
     user_id INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,