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.go

@@ -0,0 +1,27 @@
+package link
+
+import (
+	"regexp"
+	"strings"
+)
+
+var linkRe = regexp.MustCompile(`\[\[(.+?)\]\]`)
+
+func ExtractLinks(body string) []string {
+	matches := linkRe.FindAllStringSubmatch(body, -1)
+	if len(matches) == 0 {
+		return nil
+	}
+
+	seen := map[string]bool{}
+	var result []string
+	for _, m := range matches {
+		text := strings.TrimSpace(m[1])
+		if text == "" || seen[text] {
+			continue
+		}
+		seen[text] = true
+		result = append(result, text)
+	}
+	return result
+}