enabled WAL mode, pooling, pagination

WAL + pooling:
- Enable WAL journal mode for concurrent read/write support
- Add connection pool (size 4) with return_db() to reuse connections
  instead of opening/closing on every request

Pagination:
- Search results, /pages, and /tags/<name> now paginate at 50 per page
- Prev/next navigation links appear when results exceed one page

Delta sync:
- Pages table gains last_modified timestamp, set on insert/update
- /api/sites accepts ?since= param to return only changed pages
- Subscription sync uses last_sync timestamp for incremental fetches
- Remote pages upserted instead of delete-all/re-insert
- Full sync includes all_urls list for detecting remote deletions

db.py

@@ -77,13 +77,35 @@ def clean_url(url):
     return urlunparse((scheme, netloc, path, "", new_query, ""))
 
 
+_pool = []
+_pool_lock = __import__("threading").Lock()
+_POOL_SIZE = 4
+
+
 def get_db():
-    db = sqlite3.connect(DATABASE)
+    with _pool_lock:
+        if _pool:
+            db = _pool.pop()
+            try:
+                db.execute("SELECT 1")
+                return db
+            except Exception:
+                pass
+    db = sqlite3.connect(DATABASE, timeout=10)
+    db.execute("PRAGMA journal_mode=WAL")
     db.execute("PRAGMA foreign_keys = ON")
     db.row_factory = sqlite3.Row
     return db
 
 
+def return_db(db):
+    with _pool_lock:
+        if len(_pool) < _POOL_SIZE:
+            _pool.append(db)
+        else:
+            db.close()
+
+
 def init_db():
     db = sqlite3.connect(DATABASE)
     db.execute(
@@ -92,7 +114,8 @@ def init_db():
         "  url TEXT UNIQUE NOT NULL,"
         "  title TEXT,"
         "  body TEXT,"
-        "  note TEXT DEFAULT ''"
+        "  note TEXT DEFAULT '',"
+        "  last_modified TEXT DEFAULT (strftime('%Y-%m-%dT%H:%M:%S','now'))"
         ")"
     )
     db.execute(
@@ -196,26 +219,38 @@ def init_db():
         db.execute("ALTER TABLE remote_pages ADD COLUMN tags TEXT DEFAULT ''")
         db.commit()
 
+    # Migrate pages: add last_modified column if missing
+    page_cols = [row[1] for row in db.execute("PRAGMA table_info(pages)").fetchall()]
+    if "last_modified" not in page_cols:
+        db.execute("ALTER TABLE pages ADD COLUMN last_modified TEXT DEFAULT ''")
+        db.execute("UPDATE pages SET last_modified = strftime('%Y-%m-%dT%H:%M:%S','now') WHERE last_modified = ''")
+        db.commit()
+
+    db.execute("PRAGMA journal_mode=WAL")
     db.commit()
     db.close()
 
 
 def get_setting(key, default=""):
     db = get_db()
-    row = db.execute("SELECT value FROM settings WHERE key = ?", (key,)).fetchone()
-    db.close()
-    return row["value"] if row else default
+    try:
+        row = db.execute("SELECT value FROM settings WHERE key = ?", (key,)).fetchone()
+        return row["value"] if row else default
+    finally:
+        return_db(db)
 
 
 def set_setting(key, value):
     db = get_db()
-    db.execute(
-        "INSERT INTO settings (key, value) VALUES (?, ?) "
-        "ON CONFLICT(key) DO UPDATE SET value=excluded.value",
-        (key, value),
-    )
-    db.commit()
-    db.close()
+    try:
+        db.execute(
+            "INSERT INTO settings (key, value) VALUES (?, ?) "
+            "ON CONFLICT(key) DO UPDATE SET value=excluded.value",
+            (key, value),
+        )
+        db.commit()
+    finally:
+        return_db(db)
 
 
 def get_site_name():
@@ -273,10 +308,12 @@ def index_url(url, note=""):
     title, body, links = fetch_page(url)
     db = get_db()
     try:
+        now = __import__("datetime").datetime.now().strftime("%Y-%m-%dT%H:%M:%S")
         db.execute(
-            "INSERT INTO pages (url, title, body, note) VALUES (?, ?, ?, ?) "
-            "ON CONFLICT(url) DO UPDATE SET title=excluded.title, body=excluded.body, note=excluded.note",
-            (url, title, body, note),
+            "INSERT INTO pages (url, title, body, note, last_modified) VALUES (?, ?, ?, ?, ?) "
+            "ON CONFLICT(url) DO UPDATE SET title=excluded.title, body=excluded.body, "
+            "note=excluded.note, last_modified=excluded.last_modified",
+            (url, title, body, note, now),
         )
         page_id = db.execute("SELECT id FROM pages WHERE url = ?", (url,)).fetchone()[0]
         db.execute("DELETE FROM links WHERE page_id = ?", (page_id,))
@@ -287,5 +324,5 @@ def index_url(url, note=""):
             )
         db.commit()
     finally:
-        db.close()
+        return_db(db)
     return title