Make the HSTS max-age configurable with HSTS_MAX_AGE (closes #271)

HSTS_MAX_AGE, a Go duration, sets the max-age of the
Strict-Transport-Security header. Unset, it is ten years (87600h);
0 sends no header at all. A negative value, or one that is not a
whole number of seconds, stops startup like an unparseable one.
includeSubDomains and preload stay in the header. The value is shown
on /settings and in the startup log, and the README's HSTS section
now says what each part of the header commits the operator to.

Model: opus-5-5

README.md

@@ -895,10 +895,9 @@ Strict-Transport-Security: max-age=315360000; includeSubDomains; preload
 ```
 
 `HSTS_MAX_AGE` sets `max-age`, in seconds. `includeSubDomains` and `preload` are
-always part of the header. A browser that receives the header over HTTPS refuses
+always part of the header. A browser that sees the header refuses plaintext HTTP
-plaintext HTTP to that hostname until `max-age` runs out, whatever else is
+to that hostname until `max-age` runs out, whatever else is served there, so
-served there, so terminate TLS on a hostname you are prepared to keep on HTTPS.
+terminate TLS on a hostname you are prepared to keep on HTTPS.
-Browsers ignore the header when it arrives over plain HTTP.
 
 `includeSubDomains` extends this to every subdomain of the hostname webhooker is
 served at. Served at `hooks.example.com`, it covers every name under
@@ -912,10 +911,10 @@ every browser that ships the list, even for users who have never visited it, and
 removal takes months to reach those browsers, so treat a listing as permanent.
 
 Setting `HSTS_MAX_AGE=0` later stops the header being sent, but a browser that
-already received it over HTTPS does not forget it. That browser keeps refusing
+already saw it does not forget it. That browser keeps refusing plaintext HTTP
-plaintext HTTP until the `max-age` it saw runs out. Only a `max-age=0` header
+until the `max-age` it saw runs out. Only a `max-age=0` header received over
-received over HTTPS clears it sooner. webhooker never sends one; a reverse proxy
+HTTPS clears it sooner. webhooker never sends one; a reverse proxy in front of
-in front of it can.
+it can.
 
 ## Backup, Restore, and Upgrades
 
@@ -964,13 +963,6 @@ 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.