refactor: remove leaderboard, leagues, and XP progression

[FelixTJDietrich]

Problem

The leaderboard/league/XP subsystem was superseded by the practice-centric model and is flag-gated off, but its code, schema, scheduled tasks (LeaguePointsUpdateTask, SlackWeeklyLeaderboardTask), the Slack digest publisher, and the workspace.leaderboard*/leagues/progression flags all remain. It is dead weight, an unnecessary tenancy/egress surface, and confusing to contributors.

What

• Remove leaderboard/league/XP code paths (server + webapp), scheduled tasks, and the Slack digest publisher.
• Remove the related workspace feature flags and columns (including the leaderboardNotificationEnabled residue).
• Liquibase migration dropping tables/columns after a data snapshot; ERD and OpenAPI regenerated.
• The leaderboard-flag-phase-out branch (PR feat(leaderboard): enforce the workspace flag and ship the practice report read model dark #1346) is the prepared precursor — it enforces the workspace flag and ports the read model dark; build the removal on top of it.

Sequencing

Today /w/{slug} is the leaderboard page. The replacement developer-facing practice home is Valentin's deliverable (PR #1347 serves as reference, not a merge candidate) and gates Gate 2 — the code removal itself does not wait on it. The existing production instance still shows the leaderboard to real users: coordinate this removal's deploy so they are not left with an empty product in the interim.

Concrete removal scope (from the page audit)

• LeaderboardPage route (/w/$slug/index), leaderboard components, league progress/scoring dialogs.
• ProfileHeader gamification (level badge, XP bar, league points) — and note ProfileContent imports from components/leaderboard/ (ActivityBadges, ReviewsPopover, score > 0 filter): extract/refactor these before deleting the directory (feat(webapp,server): practice-centric profile v1 (self and workspace-admin audiences) #1273 is the replacement surface).
• AdminSettingsPage: strip the Leaderboard/XP/Leagues feature toggles and the AdminLeagueSettings league-reset action (embedded remnants, not standalone pages).
• Scheduled tasks + Slack digest publisher (server side, as above); delete the ai/agents / ai/practice-detection redirect shims while touching navigation.

Scope decisions

• Achievements stay (flag-gated): the achievement/ package is self-contained — no XP/league/leaderboard coupling. Post-1.0 intent: re-evaluate achievements against the practice-centric model; either integrate or remove in v2.0.0 — this sentence is the named owner of that decision.
• AC: verify at the data level that no achievement definitions reference removed XP/league events.
• AC: the pre-drop data snapshot procedure (what, where stored, who runs it) is documented and executed before the drop migration ships.
• Docs: delete the leaderboard docs page with this change; achievements docs stay while achievements stay.

Supersedes #1336 and #1305 (closed leaderboard bugs) and completes #1249's remaining column cleanup (leaderboardNotificationEnabled); realizes the direction of the closed epic #1206. The practice-centric profile (#1273) replaces the developer-facing surface.