feat(bbs): DM-based BBS with channel-based access, multi-channel whitelist, short syntax(#v1.14.0)

Adds an offline BBS accessible via Direct Message to the node's own key.
Access is channel-based: anyone seen on a configured BBS channel is
automatically whitelisted for DM access. Channels stay clean.

- Multi-channel configuration: any combination of device channels can be
  selected; senders on any of them are auto-whitelisted
- Short syntax: !p <cat> <text> and !r [cat] alongside full !bbs syntax
- Category abbreviations computed automatically (shortest unique prefix)
- handle_channel_msg: bootstrap reply on channel + auto-whitelist sender
- handle_dm: DM entry point, checks whitelist, routes to post/read/help
- DM reply routed back to sender via command_sink
- SQLite message store with WAL mode and configurable retention

README.md

@@ -1306,7 +1306,7 @@ This project is under active development. The most common features from the offi
 
 - [x] **Cross-frequency bridge** — standalone daemon connecting two devices on different frequencies via configurable channel forwarding (see [11. Cross-Frequency Bridge](#11-cross-frequency-bridge))
 - [x] **BBS — Bulletin Board System** — offline message board with DM-based commands, category/region filtering and automatic abbreviations (see [15. BBS](#15-bbs--bulletin-board-system))
-- [x] **Observer mode** — passively monitor mesh traffic without transmitting, useful for network analysis, coverage mapping and long-term logging
+- [ ] **Observer mode** — passively monitor mesh traffic without transmitting, useful for network analysis, coverage mapping and long-term logging
 - [ ] **Room Server administration** — authenticate as admin to manage Room Server settings and users directly from the GUI
 - [ ] **Repeater management** — connect to repeater nodes to view status and adjust configuration