Remove dead code flagged in the code-quality review (#528)

[maltehuebner]

Addresses item 4 (dead/unreachable code) of #528 — the fully verifiable part.

Changes

• Deleted assets/js/limit-violation.js. It references a global exceedanceData that is defined nowhere, is not a Webpack entry (webpack.config.js) and is rendered by no template. Confirmed by grep across assets/, templates/, config/, webpack.config.js and package.json.
• Removed StationRepository::findIndexedByProvider. Grep across src/ shows only the definition — no callers, not HTTP-reachable.

Deliberately not implemented / deferred

These three items are behavioural changes on DB/HTTP paths that cannot be exercised or verified in this environment (no PostGIS/Redis, and the local vendor/ lacks symfony/runtime so the kernel cannot boot). Committing them unverified would risk changing API behaviour blindly:

• N+1 queries in CityController::showAction — consolidating the per-station PollutionDataFactory native queries into one station_id IN (...) query requires DB verification of the grouped result and query-count profiling.
• Silent UniqueConstraintViolationException suppression in PostgisPersister::persistValues — changing this alters API success/failure semantics (currently returns 200) and needs DB-backed batch-insert testing to get the upsert/clear behaviour right.
• Missing value validation in PUT /api/value (value bounds, future date_time, tag > 32 chars) — a 400-vs-500 behavioural change on an endpoint that needs a bootable kernel + DB to test end-to-end.

Verification

• vendor/bin/phpstan analyse --no-progress — No errors
• vendor/bin/phpunit --testsuite="Project Test Suite" — OK (121 tests)

Refs #528 (partial)

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