feat(10-01): add pros/cons columns through backend

- Add pros/cons nullable TEXT columns to threadCandidates schema
- Generate and apply Drizzle migration (0004_soft_synch.sql)
- Mirror pros/cons columns in test helper CREATE TABLE
- createCandidate: pass pros/cons to values() object
- updateCandidate: add pros/cons to Partial type
- getThreadWithCandidates: include pros/cons in select projection
- createCandidateSchema: add optional pros/cons string fields

tests/helpers/db.ts

@@ -55,6 +55,8 @@ export function createTestDb() {
       product_url TEXT,
       image_filename TEXT,
       status TEXT NOT NULL DEFAULT 'researching',
+      pros TEXT,
+      cons TEXT,
       created_at INTEGER NOT NULL DEFAULT (unixepoch()),
       updated_at INTEGER NOT NULL DEFAULT (unixepoch())
     )