Stage 5: on-call schedule

- Migration 005: schedule_entries table (date TEXT UNIQUE, one person per day)
- POST /api/schedule — assign user to one or more dates in a single
  transaction; any date conflict rejects the whole request (409)
- GET /api/schedule — list all entries ordered by date, optional ?from/?to
- GET /api/schedule/current — today's on-call user (UTC date), 404 if none
- DELETE /api/schedule/{id} — remove an entry (204)

internal/db/migrations/005_schedule.sql

@@ -0,0 +1,8 @@
+CREATE TABLE schedule_entries (
+    id         INTEGER PRIMARY KEY AUTOINCREMENT,
+    user_id    INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    date       TEXT    NOT NULL UNIQUE,  -- YYYY-MM-DD; one person per day
+    created_at INTEGER NOT NULL DEFAULT (strftime('%s', 'now'))
+);
+
+CREATE INDEX schedule_entries_date_idx ON schedule_entries(date);