chore(server,webapp): delete ELO league-points and weekly Slack top-3 task

[FelixTJDietrich]

Part of #1206.

What ships

One cleanup PR that removes both halves of the current ranking mechanic — the ELO league-points subsystem and the weekly Slack "top 3 reviewers" broadcast. Replacement surfaces (#1272, #1273) read from practice_finding directly without any ELO-derived input.

Server-side deletions (Java, application-server):

• LeaguePointsService, LeaguePointsRecalculationService, LeaguePointsUpdateTask, LeagueChangeDTO, LeaguePointsConstants
• LeaderboardSortType.LEAGUE_POINTS enum constant (the enum itself is removed by refactor(server): extract ProfileActivityService from leaderboard internals #1272's API refactor; this sub-issue removes the constant + every reference)
• LeaguePointsRecalculator SPI under workspace/
• SlackWeeklyLeaderboardTask (Java class) and its @Scheduled registration
• The global allowlist LeaderboardProperties.workspaceSlugs (field, config binding, application.yml entry)
• Tests for all deleted classes (deleted, not commented out)

Server-side schema (delegated to #1277, called out here):

• workspace_membership.league_points, Workspace.leaderboard_schedule_*, Workspace.leaderboard_notification_* columns are dropped in fix(server): server-side feature-flag enforcement on dashboard endpoints #1277's migration; this sub-issue removes Java readers so columns have no remaining reader by the time fix(server): server-side feature-flag enforcement on dashboard endpoints #1277 drops them.

Webapp (TypeScript) deletions:

• LeagueIcon component + assets
• ScoringExplanationDialog component
• ELO-display components on leaderboard / profile rendering rank, league points, league-change tooltips

API contract:

• Existing LeaderboardController routes retain their paths for this sub-issue's duration (full surface deletion is refactor(server): extract ProfileActivityService from leaderboard internals #1272's scope); responses no longer include leaguePoints, leagueChange, or rank fields produced by deleted services. A response-shape regression test asserts the absent fields.

The SlackMessageService from #1246 is not deleted — it remains as a per-workspace utility used by other Slack-side surfaces. Only the leaderboard-task path through it is removed.

Why

The leaderboard transformation epic's foundational decision is that ELO ranking + public weekly broadcasts contradict the SDT-aligned formative-feedback mentor thesis the rest of the product is built around. Deleting both mechanisms — not flagging them off — is the right operational move: a flag-gated leaderboard rots and leaks (the existing WorkspaceFeatures.leaderboardEnabled is webapp-only and the API leaks the data regardless, captured as a separate fix in #1276); deleted code cannot leak. Combining both deletions into one PR keeps the cleanup atomic — the rest of the epic depends on the namespace being clear.

Acceptance criteria

• Every class listed above is deleted from server/application-server/src/main/java/de/tum/in/www1/hephaestus/; build succeeds; no remaining import references the deleted symbols
• LeaderboardSortType.LEAGUE_POINTS is removed; any controller / service method that accepted it as a parameter rejects requests carrying it with a structured 400; test asserts the failure shape is helpful
• LeaguePointsRecalculator SPI is removed; implementations in feature modules are deleted or simplified to no-op stubs in the same PR
• LeaguePointsRecalculator and SlackWeeklyLeaderboardTask @Scheduled registrations are gone; Spring starts cleanly with no UnknownConfigurationProperty warning, no startup error
• LeaderboardProperties.workspaceSlugs is removed from class + application.yml; no @ConfigurationProperties binding remains
• LeagueIcon, ScoringExplanationDialog, and any ELO-display webapp components are deleted; profile page asserts no data-testid containing league is present (Playwright)
• Webapp build does not import any deleted symbol; a build-time check (tsc strict + existing import-lint) fails the PR if a dead reference remains
• All unit tests covering deleted classes are deleted; test-count delta matches deletion expectations recorded in the PR description
• An ArchUnit test rejects any new code referencing deleted class names under de.tum.in.www1.hephaestus (covers LeaguePoints*, LeagueChange*, LeaguePointsConstants, SlackWeeklyLeaderboardTask)
• An integration test exercises a non-leaderboard Slack message path and asserts SlackMessageService still works (the survivor is intact)
• CHANGELOG entry under "Breaking changes" documents that workspace API responses no longer carry leaguePoints / leagueChange / rank fields, and that the weekly top-3 reviewers broadcast is removed; release notes frame the change as "replacing the broadcast with per-user reflection" via feat(server): MentorReflectionScheduler with per-user cadence and time of day #1258

Tests to write

• Compile-time + ArchUnit-time assertions are the bulk of the test work.
• Regression integration test for LeaderboardController endpoints asserting response shapes are stripped of deleted fields.
• Webapp Playwright spec on the profile page asserting no league icon / score badge renders.
• Spring-startup cleanliness test (no warnings / errors related to removed bindings).
• SlackMessageService survival assertion.

Implementation notes

• The replacement surfaces depend on ProfileActivityService (refactor(server): extract ProfileActivityService from leaderboard internals #1272) being available. This sub-issue's deletion clears the namespace before extraction; sequencing chain puts this first.
• workspace_membership.league_points and Workspace.leaderboard_* columns live until fix(server): server-side feature-flag enforcement on dashboard endpoints #1277; this sub-issue stops writing/reading them. Columns become orphaned but present until migration.
• application.yml properties under hephaestus.leaderboard.league-points.* are removed; remaining LeaderboardProperties.workspaceSlugs is handled here.
• In-flight Liquibase changesets that referenced league_points (e.g. historical seed) keep their existing rows; the column drop in fix(server): server-side feature-flag enforcement on dashboard endpoints #1277 snapshots populated values to the integration audit log (feat(server): append-only integration audit log #1218) before dropping.
• Database fixtures / seeds referencing deleted constants are updated to use LeaderboardSortType.XP_TOTAL (or its successor in the new contract) as default during transition.
• The weekly task previously closed Leaderboard Slack notifications fan out to all workspaces on a single global channel #1050 via PR fix(leaderboard): use workspace-specific Slack notification settings #771's half-migration; that issue is closed by fix(server): route weekly leaderboard Slack notifications per workspace #1246's per-workspace migration first (proving deletion is safe), then this sub-issue deletes the task entirely.
• The proactive reflection DM cadence from feat(server): MentorReflectionScheduler with per-user cadence and time of day #1258 + feat(server): shared notification budget across mentor and finding delivery #1260 is the successor surface; deliberate release-notes framing.

Dependencies

Depends on #1246 shipping first (so #1050 is closed by routing, not deletion). Blocks #1272, #1277.