vault: files sealed under a password that is never stored

Argon2id turns the vault password into a key that seals a random master
key; only the sealed copy is kept. Each file gets its own key, sealed
under the master key in its header with its media id, and its bytes are
AES-256-GCM STREAM records, so a swapped, cut or altered file refuses
to open. Uploads with ?encrypt=true are sealed as they stream in.

:vault, :unlock, :lock, :passwd, :encrypt, :decrypt and :upload! drive
it. The vault locks after WORKSPACE_VAULT_IDLE_MIN idle minutes, on
logout and on restart; while locked, sealed files answer 423, share
links never serve them, and :export! refuses rather than leave them out.

Sealing an existing file rewrites it under a new id and retires the old
one: the mirror deletes the plaintext from Proton Drive outright,
trash included, whatever WORKSPACE_BACKUP_PRUNE says, and drops it from
the readable files/ view. The mirror now compares on-disk sizes, so
sealed files are not re-hashed every run.

migrations/0012_vault.sql

@@ -0,0 +1,25 @@
+-- The vault: a password that is never stored, unlocking a master key that
+-- seals files at rest. One row at most. The master key is kept here only
+-- sealed under a key Argon2id derives from the password, with the costs it was
+-- derived at, so raising them later does not lock an existing vault out.
+CREATE TABLE vault (
+    id         INTEGER PRIMARY KEY CHECK (id = 1),
+    salt       BLOB    NOT NULL,
+    m_cost     INTEGER NOT NULL,
+    t_cost     INTEGER NOT NULL,
+    p_cost     INTEGER NOT NULL,
+    nonce      BLOB    NOT NULL,
+    sealed     BLOB    NOT NULL,
+    created_at INTEGER NOT NULL
+);
+
+-- A sealed file's bytes on disk are ciphertext; `size` stays the size of what
+-- was uploaded, which is what a download of it is.
+ALTER TABLE media ADD COLUMN encrypted INTEGER NOT NULL DEFAULT 0;
+
+-- Remote copies the mirror has to take off Drive whatever
+-- WORKSPACE_BACKUP_PRUNE says: the plaintext of a file that has since been
+-- sealed. Keeping it would undo the sealing for anyone who reads the mirror.
+CREATE TABLE backup_retired (
+    path TEXT PRIMARY KEY
+) WITHOUT ROWID;