fix(backup): store a backup in sealed pieces instead of one value

A stored backup was one value in data_backups.data. v1.39.1 wrote it a
piece at a time but still joined the pieces into that one value, and one
value has to pass through the app whole, so a copy was capped at a fifth of
the heap: 105 MB in the default 1 GB container. An account with 1.75
million readings is past that and could not be backed up at all (#1031).

A copy is now kept as ordered pieces of about a megabyte in the new
data_backup_chunks table (migration 0352). Each piece is sealed on its own
with AES-256-GCM, and its sealed header names the copy (a random stream id
kept on the row), its position and whether it is the last. A reader checks
every piece before it releases a byte, so a piece that is missing, moved,
altered, taken from another copy, or a copy cut short is refused with
backup.payload.undecryptable before a restore deletes anything, and the
second read during the restore checks each piece again.

The write is one transaction, so the previous copy stays whole and readable
until the new one commits. Pieces are written through the model API: a raw
query with a large parameter is kept alive by Prisma's query plan cache,
which left 160 MB of heap behind per hundred one-megabyte inserts.

Restore, preview, download, the restore job and the admin list read both
forms; copies written before this change keep their single value and still
restore. The heap-derived limit is gone. The one limit left is on storage,
BACKUP_MAX_STORED_MB (default 2048), and its message names the setting. Key
rotation covers the pieces, the wipe and account deletion take them with
their backup, and the backup plan lists the table as not carried.

Measured on an account of 2.6 million readings in a 1 GB container with web
and worker in one process: backup 87 s, 97 MiB in 96 pieces, preview 20 s,
download 25 s, restore 264 s with every reading back exactly, peak 567 MiB,
no restart.

.env.example

@@ -234,6 +234,17 @@ API_TOKEN_HMAC_KEY="REPLACE-WITH-OUTPUT-OF-openssl-rand-hex-32"
 # BACKUP_RETENTION_DAYS="30"        # the number for your bucket lifecycle rule (see above)
 # BACKUP_ENCRYPTION_KEY=""          # 64-hex-char or base64 32-byte; SEPARATE from ENCRYPTION_KEY
 
+# -----------------------------------------------------------------------------
+# Stored backups in the database (optional)
+# -----------------------------------------------------------------------------
+# The largest weekly or uploaded backup of one account the app keeps, in
+# megabytes of encrypted copy. Default 2048. It does not depend on the app's
+# memory: a copy is written and read in pieces of about a megabyte. The limit
+# only stops a runaway copy from filling the database volume, which holds the
+# old and the new copy side by side while a backup runs. An account with 2.6
+# million readings stores a copy of about 100 MB.
+# BACKUP_MAX_STORED_MB="2048"
+
 
 # -----------------------------------------------------------------------------
 # Audit-trail retention (optional)