Phase 4: per-child sessions, scan both children, items separated

Fetch and parse Harry and Finn with a stored Compass session per child and
store every item against the right children.id.

- cmd/school/session.go + internal/store/session.go: read and write the
  per-child harness session state in the SQLite `session` table. `show`
  prints metadata only; the state is never printed.
- scripts/compass-fetch-patchright.mjs: reuse the stored session, refresh it
  after login, resolve the child's userId from the parent page by name, and
  keep the shared calendar out of the per-child run (it would leak the
  eldest's timetable into the younger child).
- scripts/scan-compass.sh + make scan: scan one child or both; restore the
  session, fetch, persist the session, then parse. A failing child is
  recorded in `scans` with ok = 0 and the script exits non-zero (FR-12).
- cmd/school/scan.go: `school scan record` writes a fetch failure to the
  scans table. cmd/school/items.go: `school items purge` removes rows that a
  shared page stored against the wrong child.
- internal/store/store_test.go: prove items and sessions are separated by
  child. docs/phase-4-both-children-notes.md; README + image tag 0.3.0-phase4.

Live evidence: Harry 70 tasks / 1 message / 35 events (6 overdue); Finn
42 tasks / 1 message / 0 events (2 overdue). Finn's subjects are Year 7,
Harry's are Year 11. Both sessions stored.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

cmd/school/items.go

@@ -0,0 +1,70 @@
+package main
+
+import (
+	"context"
+	"flag"
+	"fmt"
+	"os"
+
+	"gitea.lab.audasmedia.com.au/sam/compass_teams_nhs/internal/config"
+	"gitea.lab.audasmedia.com.au/sam/compass_teams_nhs/internal/store"
+)
+
+// itemsCommand is the `school items` entry point. It currently supports purging
+// items for one child, which Phase 4 uses to drop rows that a shared page
+// stored against the wrong child ("nothing may leak between children").
+func itemsCommand(args []string) {
+	if len(args) == 0 || args[0] != "purge" {
+		fmt.Fprintln(os.Stderr, "usage: school items purge --child NAME [--source compass|teams] [--type message|task|event]")
+		os.Exit(2)
+	}
+	fs := flag.NewFlagSet("items purge", flag.ContinueOnError)
+	var (
+		child  string
+		source string
+		typ    string
+	)
+	fs.StringVar(&child, "child", "", "child name (Harry or Finn)")
+	fs.StringVar(&source, "source", "", "limit to one source")
+	fs.StringVar(&typ, "type", "", "limit to one type")
+	_ = fs.Parse(args[1:])
+
+	if child == "" {
+		fmt.Fprintln(os.Stderr, "school items purge: --child is required")
+		os.Exit(2)
+	}
+	if err := runItemsPurge(child, source, typ); err != nil {
+		fmt.Fprintln(os.Stderr, "school items:", err)
+		os.Exit(1)
+	}
+}
+
+func runItemsPurge(child, source, typ string) error {
+	ctx := context.Background()
+
+	cfg, err := config.LoadDB()
+	if err != nil {
+		return err
+	}
+	st, err := store.Open(cfg.DBPath)
+	if err != nil {
+		return err
+	}
+	defer st.Close()
+	if err := st.Migrate(ctx); err != nil {
+		return err
+	}
+	c, err := st.ChildByName(ctx, child)
+	if err != nil {
+		return err
+	}
+	if c == nil {
+		return fmt.Errorf("unknown child %q", child)
+	}
+	n, err := st.DeleteItems(ctx, c.ID, source, typ)
+	if err != nil {
+		return err
+	}
+	fmt.Printf("child=%s purged=%d source=%q type=%q\n", c.Name, n, source, typ)
+	return nil
+}