add Anope->Echo migration importer (echo import)

New 'echo import <anope.json> <out.log> [node]' subcommand builds a fresh event
log from an Anope db_json: accounts (identity only — Anope's one-way password
hashes can't become SCRAM verifiers, so migrated users auth by certfp/external
authority), grouped nicks, cert fingerprints, vhosts, channels (+desc/topic/
assigned bot), access mapped to op/voice by each channel's AUTOOP/AUTOVOICE
thresholds, param-less mode locks, BotServ bots (service pseudo-clients skipped),
and memos. Read-only on the source; self-verifies by replaying the produced log.
Verified against the live backup (11 accounts, 10 channels, 10 bots) + a unit
test over a synthetic fixture. The prerequisite for a safe cutover.

src/engine/db/mod.rs

@@ -1202,6 +1202,13 @@ impl Db {
         Ok(())
     }
 
+    /// Append a pre-built event during a one-off import, bypassing the live-mutation
+    /// methods so original timestamps and ids are preserved verbatim. State is
+    /// rebuilt from the log on the next open; only meant for the `import` tool.
+    pub fn migrate_append(&mut self, event: Event) -> std::io::Result<()> {
+        self.log.append(event)
+    }
+
     /// Whether the log has grown enough past the live state to be worth compacting.
     pub fn should_compact(&self) -> bool {
         self.log.len() > (self.accounts.len() + self.channels.len()) * 3 + 64