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

README.md

@@ -693,11 +693,13 @@ Key settings in more detail:
 - `signing_key` — HMAC secret for URL signatures
 - `db_url` — the SQLite database to open; omitted, it is
   `file:<state_dir>/state.sqlite3?_pragma=journal_mode(WAL)`, which keeps the
-  database in WAL mode. pixa opens one connection to it, so its own reads and
-  writes run one at a time. pixa adds `_pragma=busy_timeout(5000)` to any
-  `db_url`, so a write that finds another program writing to the file waits up
-  to five seconds for it instead of failing. WAL mode comes only from the URL:
-  keep `_pragma=journal_mode(WAL)` in one you set
+  database in WAL mode. `state_dir` is percent-encoded in it, so a `%`, `?` or
+  `#` in the name is written as `%25`, `%3F` or `%23`. pixa opens one connection
+  to it, so its own reads and writes run one at a time. pixa adds
+  `_pragma=busy_timeout(5000)` to any `db_url`, so a write that finds another
+  program writing to the file waits up to five seconds for it instead of
+  failing. WAL mode comes only from the URL: keep `_pragma=journal_mode(WAL)` in
+  one you set
 - `cache_max_bytes` — disk cache size limit in bytes; `0` disables the disk
   cache entirely; omitted defaults to 75% of the sum of the free space on the
   filesystem containing `<state_dir>/cache/` and the bytes of source and