Harden network and privacy defaults; fix several bugs

Security:
- Bind HTTP gateway to 127.0.0.1 by default; add --bind for LAN opt-in
- Restrict Reticulum mesh surface to GET /api/sites only (CSRF cannot
  authenticate mesh callers, so gate by whitelist)
- Cap request body size at 16 MiB to prevent memory DoS
- Redact /bookmark query strings from request logs so the bookmark token
  and URLs do not land in stdout / docker / journal logs
- Tighten FTS5 sanitizer: strip colon, drop AND/OR/NOT/NEAR operator words
- Expand .dockerignore; document trust model in README

Features:
- Add sharing mode toggle (share everything except private vs share only
  public-tagged) with /share/preview so users can see what subscribers
  would receive before enabling sharing

Bugs:
- handle_export() crashed on every call (missing query kwarg)
- Dead float16 decompression branch in embeddings.py silently corrupted
  the HNSW index when compress_embeddings was on
- GATEWAY_PORT staleness: --port and find_available_port had no effect
  on the actual bind
- semantic_search default mismatched between db.py ("1") and the rest of
  the app ("0"), causing embeddings to be generated when the UI said off
- Connection pool returned connections with uncommitted transactions to
  the next consumer
- Gateway POST body decode 502'd on non-UTF-8 input
- ensure_rns_config clobbered user-edited ~/.reticulum/config; now only
  rewrites files it authored (sentinel-tagged)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

db.py

@@ -123,6 +123,14 @@ def get_db():
 
 
 def return_db(db):
+    try:
+        db.rollback()
+    except Exception:
+        try:
+            db.close()
+        except Exception:
+            pass
+        return
     with _pool_lock:
         if len(_pool) < _POOL_SIZE:
             _pool.append(db)
@@ -412,7 +420,7 @@ def index_url(url, note="", reticulum_dest=""):
                 (page_id, href, label),
             )
         db.commit()
-        if get_setting("semantic_search", "1") == "1":
+        if get_setting("semantic_search", "0") == "1":
             try:
                 from embeddings import store_embeddings
                 store_embeddings(page_id, title, body, db)