feat(graph,graphstore): M2 parse tier+topic from frontmatter, persist via Upsert (infra#72)

extract.go now reads `tier:` and `topic:` from YAML frontmatter, with
a path-based fallback when frontmatter is absent (the pre-M3 state on
every existing entry):

  knowledge/* → tier=knowledge
  notes/*     → tier=note
  wiki/**     → tier=note   (sources + concepts + entities are I-level)
  inbox/**, raw/**, sessions/**, clips/** → tier=inbox

Frontmatter wins when present — covers the M3-migrated case where an
entry's path may not match the tier the author chose for it.

UpsertEntity persists both columns. M1's schema already has them.

Backfill on next pod start populates tier for the whole corpus
without any file moves; M3 will follow up with the actual layout
migration and explicit frontmatter writes.

ingestion/internal/graph/extract_test.go

@@ -119,6 +119,51 @@ func TestExtract_WikiRootFileIsKnowledgeNotHall(t *testing.T) {
 	assert.Empty(t, ent.Hall)
 }
 
+func TestExtract_TierFromFrontmatter(t *testing.T) {
+	content := []byte(`---
+tier: knowledge
+topic: postgres-roles
+title: Least-privilege migration trap
+---
+# body
+`)
+	ent, _, ok := Extract("knowledge/some-lesson.md", content)
+	require.True(t, ok)
+	assert.Equal(t, "knowledge", ent.Tier)
+	assert.Equal(t, "postgres-roles", ent.Topic)
+}
+
+func TestExtract_TierInferredFromPath(t *testing.T) {
+	cases := []struct {
+		path string
+		want string
+	}{
+		{"knowledge/foo.md", "knowledge"},
+		{"wiki/sources/x.md", "note"},
+		{"wiki/concepts/x.md", "note"},
+		{"wiki/x.md", "note"},
+		{"inbox/clips/x.md", "inbox"},
+		{"notes/x.md", "note"},
+		{"raw/x.md", "inbox"},
+		{"sessions/x.md", "inbox"},
+	}
+	for _, tc := range cases {
+		ent, _, ok := Extract(tc.path, []byte("# x\n"))
+		require.True(t, ok, tc.path)
+		assert.Equal(t, tc.want, ent.Tier, tc.path)
+	}
+}
+
+func TestExtract_FrontmatterTierBeatsPathInference(t *testing.T) {
+	// A clip explicitly promoted via frontmatter wins over the path's
+	// inbox inference. Catches the case where a file has been moved
+	// to a new location but frontmatter hasn't been updated.
+	content := []byte("---\ntier: knowledge\n---\n# x\n")
+	ent, _, ok := Extract("inbox/clips/x.md", content)
+	require.True(t, ok)
+	assert.Equal(t, "knowledge", ent.Tier)
+}
+
 func TestExtract_WikiRootFileWithFrontmatterWingHall(t *testing.T) {
 	content := []byte(`---
 wing: homelab