feat: watch BookMyShow alongside District in one process

BookMyShow sits behind Cloudflare bot management that rejects curl and Node fetch
with 403 on the TLS fingerprint alone, so no header or cookie copying helps and a
proxy makes no difference. impit performs the request with a real Chrome
fingerprint; it ships musl and arm64 binaries, so the Alpine multi-arch image is
unaffected. BMS has no 204 either: a date that is not on sale returns 200 with no
showtimeWidgets key, so 'closed' is read from the payload.

Sites now live in src/providers/, each exposing check() and extract() and returning
one normalized session shape. State is namespaced per site, so each keeps its own
dedupe and failure tracking and neither can silence the other. Alert titles name the
site. Sessions without exact seat counts render without them, which BMS needs.

Old flat District variable names still work. CI now starts the built image and runs
a real check through both providers before publishing, since a broken native binary
only fails at runtime.

test/fixtures/bms-closed-date.json

@@ -0,0 +1 @@
+{"data":{"header":{"title":{"styleId":"header-title-v2","text":"Avengers Endgame: Encore"},"subtitle":{"styleId":"screen-subtitle","text":"Movie runtime: 3h 5m"},"startActions":[{"iconUrl":"https://assets-in.bmscdn.com/moviesmaster/movies-showtimes/v4/common/left-chevron.png","cta":{"type":"closeScreen","analytics":{"event_action":"back","event_name":"showtime_actions","event_type":"click"}}}],"endActions":[{"iconUrl":"https://assets-in.bmscdn.com/moviesmaster/movies-showtimes/v4/common/search.png","cta":{"type":"searchView","analytics":{"event_action":"search","event_name":"showtime_actions","event_type":"click"}}}],"backgroundColor":"#FFFFFF","searchBackgroundColor":"#FFFFFF","searchForegroundColor":"#000000","additionalData":{"genre":"Action|Adventure|Fantasy"}},"additionalData":{},"addOnWidgets":{}},"metadata":{"error":{"title":"Uh-oh! We couldn't find anything.","message":"There are no shows available in this region.","errorCode":"1a.1b.2001","imageUrl":"https://assets-in.bmscdn.com/moviesmaster/movies-showtimes/v4/errors/no-showtimes-found-eventcode.png","action":{"label":"Change Location","backgroundColor":"#EB4E62","textColor":"#FFFFFF","cta":{"type":"changeLocation"}}}}}