Add optional inbound relay (backup-MX) behind INBOUND_RELAY_ENABLE.

Port 25 accepts only configured domains and listed recipients, then forwards to an upstream; the outbound path is unchanged when the flag is off.

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/store/migrations/0006_inbound_relay.sql

@@ -0,0 +1,26 @@
+-- Optional inbound relay (backup-MX / forwarder). Separate from sending
+-- domains: these rows exist even when INBOUND_RELAY_ENABLE is false, but the
+-- listener, Postfix maps and panel UI are generated only when that flag is on.
+
+CREATE TABLE inbound_domains (
+    id              INTEGER PRIMARY KEY AUTOINCREMENT,
+    name            TEXT NOT NULL UNIQUE,
+    recipient_mode  TEXT NOT NULL CHECK (recipient_mode IN ('list', 'any')),
+    created_at      TEXT NOT NULL
+);
+
+-- One upstream per inbound domain (host:port + TLS policy for the hand-off).
+CREATE TABLE inbound_transports (
+    inbound_domain_id INTEGER PRIMARY KEY REFERENCES inbound_domains(id) ON DELETE CASCADE,
+    host              TEXT NOT NULL,
+    port              INTEGER NOT NULL CHECK (port >= 1 AND port <= 65535),
+    tls_mode          TEXT NOT NULL CHECK (tls_mode IN ('may', 'encrypt', 'none'))
+);
+
+-- Explicit recipients for recipient_mode = 'list'. Ignored when mode is 'any'.
+CREATE TABLE inbound_recipients (
+    id                INTEGER PRIMARY KEY AUTOINCREMENT,
+    inbound_domain_id INTEGER NOT NULL REFERENCES inbound_domains(id) ON DELETE CASCADE,
+    address           TEXT NOT NULL,
+    UNIQUE (inbound_domain_id, address)
+);