[LAPOINTE] Move 395 Discourse sites to the DiscourseJson engine (#3089)

Moves 395 sites from the `Discourse` engine to `DiscourseJson`, which checks
`/u/{username}.json` instead of the HTML profile page.

The JSON answer carries the fields socid_extractor already parses — uid, trust
level, badges, registration and last-seen dates: sixteen fields against four,
in the same single request.

Measured before the move, on all 464 Discourse entries: 403 serve that JSON to
a guest, and a name that cannot exist answers 404 with
`{"errors":[…],"error_type":"not_found"}`. Each of the 395 was then run through
maigret on the converted entry — the claimed name reports CLAIMED, three
impossible names report AVAILABLE.

Eleven are more accurate than before, not merely richer: on
`community.brave.com`, `forums.sketchup.com`, `community.smartthings.com` and
eight others, an account that exists answers `/u/<name>.json` with its user id
while `/u/<name>/summary` answers 404 — so the HTML engine has been reporting
real accounts as available.

Left on the HTML engine: 9 entries that answer 200 with HTML for a real member
and for an impossible name alike, 9 that answer 403, 21 that did not answer at
all, and `discuss.ardupilot.org`, the one entry where the JSON route proved
less reliable than the HTML one.

maigret/resources/db_meta.json

@@ -1,8 +1,8 @@
 {
     "version": 1,
-    "updated_at": "2026-09-06T07:13:33Z",
+    "updated_at": "2026-09-06T17:54:17Z",
     "sites_count": 4410,
     "min_maigret_version": "0.5.0",
-    "data_sha256": "daae2a6b821822cf8a7909f595fe1f1eb3fe3e64ec789d05882308e0380f7783",
+    "data_sha256": "7006c14f70255f213b952f9a5ba9c130705975fb025eb1217209b7e74e227da7",
     "data_url": "https://raw.githubusercontent.com/soxoj/maigret/main/maigret/resources/data.json"
 }