feat(db): add wiki-link extraction, resolution, and backlinks

[[wiki-links]] in entry bodies are extracted at save time, resolved
to entity IDs (title match first, body substring fallback), and
stored in entity_links junction table. Backlinks surface in TUI
detail view showing entries that link to the current entry.

Schema migration v5 adds entity_links with CASCADE/SET NULL
semantics. Links sync on Create, Update, and Absorb.

internal/link/extract_test.go

@@ -0,0 +1,38 @@
+package link
+
+import "testing"
+
+func TestExtractLinks(t *testing.T) {
+	tests := []struct {
+		name string
+		body string
+		want []string
+	}{
+		{"no links", "plain text with no links", nil},
+		{"single link", "see [[nginx config]] for details", []string{"nginx config"}},
+		{"multiple links", "see [[nginx config]] and [[deploy steps]]", []string{"nginx config", "deploy steps"}},
+		{"duplicate deduped", "[[foo]] then [[foo]] again", []string{"foo"}},
+		{"empty brackets", "empty [[ ]] ignored", nil},
+		{"just brackets no content", "[[]] empty", nil},
+		{"link with special chars", "see [[deploy: staging (v2)]]", []string{"deploy: staging (v2)"}},
+		{"link in markdown", "# heading\n\nsee [[my note]] for info", []string{"my note"}},
+		{"adjacent links", "[[one]][[two]]", []string{"one", "two"}},
+		{"partial brackets ignored", "not a [link] or [[incomplete", nil},
+		{"link with hash", "see [[#ops channel]]", []string{"#ops channel"}},
+		{"multiline body", "line one [[link one]]\nline two [[link two]]", []string{"link one", "link two"}},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := ExtractLinks(tt.body)
+			if len(got) != len(tt.want) {
+				t.Fatalf("got %v, want %v", got, tt.want)
+			}
+			for i := range got {
+				if got[i] != tt.want[i] {
+					t.Fatalf("got %v, want %v", got, tt.want)
+				}
+			}
+		})
+	}
+}