Add WAL mode, connection pooling, pagination, and delta sync

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

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

rns_client.py

@@ -6,11 +6,11 @@ ASPECTS = ["server"]
 REQUEST_TIMEOUT = 30
 
 
-def fetch_remote_sites(dest_hash_hex):
+def fetch_remote_sites(dest_hash_hex, since=""):
     """
     Connect to a remote TinyWeb instance over Reticulum and fetch its
     shared sites. Returns the response dict from /api/sites, or raises
-    an exception on failure.
+    an exception on failure. Pass `since` as ISO timestamp for delta sync.
     """
     dest_hash = bytes.fromhex(dest_hash_hex)
 
@@ -48,10 +48,11 @@ def fetch_remote_sites(dest_hash_hex):
 
     try:
         # Request /api/sites
+        query = {"since": [since]} if since else {}
         request_data = {
             "method": "GET",
             "path": "/api/sites",
-            "query": {},
+            "query": query,
             "body": {},
             "gateway_host": "",
         }