Escape state_dir in the db_url derived from it (closes #290)

The derived db_url put state_dir into a file: URL as it was, so SQLite
decoded a %HH in the name and ended the path at a ? or #, and the
journal mode parameter was lost. It is now built with net/url, which
percent-encodes the directory name; SQLite decodes it back and opens
state.sqlite3 in that directory. OmitHost keeps the usual form
file:/var/lib/pixa/state.sqlite3 and a relative state_dir a path.

Model: opus-5-5

TODO.md

@@ -62,6 +62,12 @@ P2: security: per-origin rate limiting
   another way, such as with `%2F` in the path, whatever else the path holds, or
   with a user name and password. It also answers 400 for a source path holding a
   `..` segment, as `/v1/image/` refuses one.
+- 2026-10-16 a `state_dir` whose name holds `%`, `?` or `#` keeps its database
+  at `state.sqlite3` in that directory (closes #290): the `db_url` derived from
+  it put the name into a `file:` URL as it was, so SQLite read `%41` as `A`, a
+  `?` or `#` ended the path early, and the journal mode parameter was lost. The
+  derived `db_url` is now built with Go's `net/url`, which writes those
+  characters as `%25`, `%3F` and `%23`.
 - 2026-10-15 a thin source requested with `fit=cover` no longer makes libvips
   build a huge image (closes #282): libvips' thumbnail scaled the whole source
   to cover the requested size and copied that into memory before it cut out the