Add the interests table and module (first-class interests, step 1)

Standing interests get a table of their own plus article_interests, the
per-run top-3 matches, and a pure rates() that derives each interest's
Beta-smoothed weight from current ratings the way feed affinity does.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01K9PrjtUS16PAQve8D4bHgc

migrations/0013_interests.sql

@@ -0,0 +1,18 @@
+CREATE TABLE interests (
+    id             INTEGER PRIMARY KEY AUTOINCREMENT,
+    name           TEXT NOT NULL COLLATE NOCASE UNIQUE,
+    category       TEXT,                      -- NULL until categorized
+    created_at     TEXT NOT NULL,
+    categorized_at TEXT
+);
+CREATE INDEX idx_interests_category ON interests(category);
+
+CREATE TABLE article_interests (
+    article_id  INTEGER NOT NULL REFERENCES articles(id) ON DELETE CASCADE,
+    interest_id INTEGER NOT NULL REFERENCES interests(id) ON DELETE CASCADE,
+    cos         REAL NOT NULL,
+    z           REAL NOT NULL,
+    run_id      INTEGER,                      -- NULL for backfilled rows
+    PRIMARY KEY (article_id, interest_id)
+);
+CREATE INDEX idx_article_interests_interest ON article_interests(interest_id, cos DESC);