runtime: ranges, health, logs, supervised loops, strict config

Media answers Range requests with 206 and Content-Range, so videos seek
and Safari plays them. Sealed files too: the vault's opener now decrypts
by record position (STREAM BE32 through StreamPrimitive, the same
construction and the same files), so a range starts at the record that
holds its first byte. Compression is limited to text, because a gzipped
body no longer matches the byte ranges a player asks for.

GET /api/health answers without a session once the database does, and
the compose healthcheck asks it instead of only opening the port.
Requests are logged at info with share tokens and keys left out of the
path and no query strings. A request that is not answered in 30 seconds
gets a 503, except uploads, imports, the export, re-sealing and backups.

The backup, mail, sweep and SFTP loops run under a supervisor that logs
a panic and starts the loop again. The backup loop starts without a
Proton session and picks up a later proton-login without a restart. A
lock file keeps backup-now and the server from mirroring at once, and an
unreadable session key file turns backups off instead of failing boot.

Numbers in the configuration are checked against bounds at startup, so
a session length or an upload limit can no longer overflow later, and a
switch that is neither on nor off is an error rather than off. Config
and MailConfig print with the password, secret and SMTP URL redacted.

A session epoch is signed into each cookie (migration 0014). :logout!
advances it, which ends every session, and a changed WORKSPACE_PASSWORD
advances it on the next start. Cookies issued before this keep working
in the first epoch.

migrations/0014_session_epoch.sql

@@ -0,0 +1,10 @@
+-- Sessions are signed cookies, not rows: nothing on the server lists them, so
+-- nothing can revoke one. Signing a number into every cookie that has to match
+-- this one gives a single lever instead — raise it, and every cookie issued
+-- before is refused. It is raised by `:logout!` and whenever the login
+-- password changes, which is recognised by a keyed hash of it kept here.
+CREATE TABLE auth (
+    id            INTEGER PRIMARY KEY CHECK (id = 1),
+    epoch         INTEGER NOT NULL,
+    password_hash TEXT    NOT NULL
+);