Document that DATA_DIR must be on a local filesystem (closes #295)

SQLite in WAL mode maps each database's -shm file into memory, and
network filesystems do not support that reliably, so the README now
says DATA_DIR must be local and names NFS and SMB/CIFS as unsupported.
It also says what a disk I/O error with code 522 points at and what to
try first. ZFS is not called unsupported: the one failure seen there
was never reproduced.

Model: opus-5-5

README.md

@@ -941,6 +941,13 @@ with an error naming it: for `webhooker.db` the server and `webhooker resetpw`
 stop, and an event or archive database fails as a damaged one does (see
 [Database Architecture](#database-architecture)).
 
+**`DATA_DIR` must be on a local filesystem.** Under WAL, SQLite maps each
+database's `-shm` file into memory, which network filesystems (NFS, SMB/CIFS) do
+not support reliably, so they are not supported. webhooker does not check which
+filesystem `DATA_DIR` is on. An error reading `disk I/O error (522)`
+(`SQLITE_IOERR_SHORT_READ`) points at the storage under `DATA_DIR`, not at
+locking; moving `DATA_DIR` to a plain local disk is the first thing to try.
+
 Configuration is **not** in `DATA_DIR` — it comes from the environment and from
 a `.env` file read out of the process working directory. Back that up with your
 deployment config, separately.