Add message retention and hide emails by default

- db: cleanup_old_messages(days) purges messages older than N days in
  batches; recipients/reads/deliveries/reactions follow via ON DELETE
  CASCADE. Returns attachment file_ids no longer referenced by any
  surviving message (forwarded copies keep their files) and removes
  their image_uploads rows
- server: MESSAGE_RETENTION_DAYS env var (default 0 = keep forever);
  hourly cleanup deletes expired messages and securely removes orphaned
  attachment blobs from the upload dir
- schema: email_visible now defaults to 0 — previously any logged-in
  user who knew a UUID could read another user's email via get_profile
- migrations: SQL script to apply the new default and reset the flag on
  existing databases (run manually, see file header)
- docker-compose: document MESSAGE_RETENTION_DAYS

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

schema.sql

@@ -155,7 +155,7 @@ CREATE TABLE IF NOT EXISTS user_profiles (
     user_id CHAR(36) NOT NULL PRIMARY KEY,
     phone VARCHAR(50) DEFAULT NULL,
     phone_visible TINYINT(1) NOT NULL DEFAULT 0,
-    email_visible TINYINT(1) NOT NULL DEFAULT 1,
+    email_visible TINYINT(1) NOT NULL DEFAULT 0,
     location VARCHAR(255) DEFAULT NULL,
     location_visible TINYINT(1) NOT NULL DEFAULT 0,
     avatar_file VARCHAR(255) DEFAULT NULL,