feat(pgp): add Autocrypt and automatic key discovery - #602

Builds on #595.

• Autocrypt Level 1. Two per-account switches, both off by default: send your key (Autocrypt: header with a minimised key) and encrypt by default (prefer-encrypt=mutual). Incoming headers are read when an account has it on, with addr= checked against From, and kept in a new pgp_peers table. Keys go into Hylki's own GnuPG home (discovered-keys), never the user's keyring; the reader's one-click Autocrypt import now goes there too. The full recommendation algorithm (disable, discourage, available, encrypt), and Autocrypt-Gossip inside encrypted mail to several recipients.
• Automatic key discovery. WKD over HTTPS in Rust (advanced, then direct only when the advanced host is unreachable), key must carry the looked-up address; keys.openpgp.org only when enabled. The composer looks recipients up 600 ms after they settle, caching misses for a day.
• Per-recipient indicator. A lock badge on each recipient pill (keyring, Autocrypt/WKD, gossip or stale, none), with tooltips and accessible descriptions. Encrypt turns on by itself when every recipient and the account prefer it.
• docs/OPENPGP_BACKEND.md compares GnuPG, Sequoia and RNP for RFC 9580, recommending Sequoia for the discovered-keys keyring first.

Tests: header parse/emit, the recommendation table, peer-state updates, gossip filtering, WKD URLs and fallback with a fake network, and real-gpg tests that found keys stay out of the user's keyring and forged headers are refused. Not tested live: WKD against real servers.