weekly challenges: admin-configurable slate + working skin rewards

Port the admin-editable weekly-challenge feature to the 2020.03 branch
and fix the skin-reward bug it had on December: rewards now actually
land in the player's inventory instead of only granting XP + tokens.

Server config: add WeeklyChallenges* columns to ServerSettings (Postgres
post-EnsureCreated patches + SQLite column patches), and port the
ServerSettingsService weekly methods/records. The reward record carries
the store item Slug — the reliable handle December lacked — so the grant
can resolve the real item.

ProgressionController: CurrentChallenge + updateProgress now read the
configured slate and reward instead of a hardcoded one, keeping the
2020.03 ChallengeMap/ChallengeGift wire shapes (verified against the
march dump: ChallengeGift TypeDefIndex 11455). The Gift now sends
AvatarItemType (Outfit when an AvatarItemDesc is set) so the watch's
gift-card render binds the avatar preview. On completing the week's
challenges the configured skin/consumable is granted via the new
StoreService.GrantItemFreeBySlugAsync (same inventory path as a purchase,
no charge), gated behind the once-only reward sentinel.

Admin: GET/POST settings/weekly-challenges + reward-options (avatar +
consumable only — 2020.03 StoreService has no equipment payload helper),
and the admin-ui Server settings page to edit the slate + reward. The
reward picker keys on item slug rather than December's fragile
descriptor-string match.

Docs: README no longer lists weekly challenges as December-only;
url-coverage notes the config-driven challenge endpoints + skin grant.

README.md

@@ -28,11 +28,16 @@ Subsystems **NOT** present on this branch (these landed in December):
 - Clubs surface
 - Playlists (curated + user-created)
 - Group DMs / chat threads
-- Weekly challenges
 - Room keys (purchasable room access)
 - Per-room co-owner / moderator / host roles
 - Late-2020 store catalog additions
 
+Weekly challenges **are** present on this branch: the admin SPA's
+**Server settings** page edits the weekly slate, the `CompletedRequired`
+flag, and the gift (XP + tokens, plus an optional store skin/consumable
+that's granted straight to the player's inventory when the week's
+challenges complete).
+
 If you need any of those, you're on the wrong branch — see
 [`december-2020-12-18`](../../tree/december-2020-12-18).