practicas filter-dropdown perf: getPracticasFilterOptions 2.2s/call under load

dbperf snapshot flagged json_agg subquery mean=2212ms/76 calls, +10619ms total. Source: getPracticasFilterOptions (src/lib/practica.ts) — JTP picker off vStudentTeacherRoster rebuilds full picks CTE (seq scan all practicas + window funcs + nested joins) on every /practicas filter-dropdown load. Fix: wrapped in unstable_cache (per-user key userId+fullAccess, 5min TTL), no DB change. Deviation from db's option-(a) rec: no revalidateTag wired (repo's revalidateTag now needs a 2-arg tag+profile signature nothing else in the codebase uses yet; both existing caches are TTL-only precedent; stale dropdown options are cosmetic since ?jtp= filters server-side regardless). Tag declared on the cache for future adoption.