Threading: Antworten (reply_to) + Focus-View

- entry: reply_to/reply_count/last_activity; reply_count bubbelt bis Root
- Hauptfeed nur Roots nach last_activity, Profil-Feed inkl. Antworten
- GET /entry/{pid}/thread (Ahnen+Antworten), GET /e/{pid} Focus-Seite
- Frontend: Antworten-Link im Feed, entry.html/entry.js Focus-Seite
- notes/migrations.md: ALTER TABLE fuer Prod

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

db.go

@@ -28,12 +28,15 @@ CREATE TABLE IF NOT EXISTS session (
 );
 
 CREATE TABLE IF NOT EXISTS entry (
-    id         INTEGER PRIMARY KEY AUTOINCREMENT,
-    pid        INTEGER UNIQUE,
-    uid        INTEGER,
-    created_at INTEGER,
-    content    TEXT,
-    filepath   TEXT
+    id            INTEGER PRIMARY KEY AUTOINCREMENT,
+    pid           INTEGER UNIQUE,
+    uid           INTEGER,
+    created_at    INTEGER,
+    content       TEXT,
+    filepath      TEXT,
+    reply_to      INTEGER NOT NULL DEFAULT 0,
+    reply_count   INTEGER NOT NULL DEFAULT 0,
+    last_activity INTEGER
 );
 
 CREATE TABLE IF NOT EXISTS vote (