feat(defects): read API — feed, per-section counts and history (T6)

GET /api/defects returns a newest-first feed joined to the project name,
bounded by limit; ?section=ID narrows to one section's history. GET
/api/defects/counts returns { sectionId: n } for the weak-point map. Adds a
(section_id, created_at) index and a db:seed script; ~2k rows query in ~20ms.

server/db/migrations/0002_mixed_chronomancer.sql

@@ -0,0 +1 @@
+CREATE INDEX "defects_section_created_idx" ON "defects" USING btree ("section_id","created_at" DESC NULLS LAST);