[DT-4061] Add similar and frequently-requested-with study recommendations

[otchet-broad]

Base: branch 4 · 11 files, +447 · Migration: no · UI impact: none

Adds GET /api/metrics/study-recommendations/{studyId}/similar and GET /api/metrics/study-recommendations/{studyId}/frequently-requested-with. Both
return at most 12 publicly visible studies with their dataset counts.

"Similar" matches on a shared PI or at least one overlapping data type,
ranked by the size of the data type overlap plus a point for the PI match. A
blank pi_name is not an identity, so it never matches another blank one.

"Frequently requested with" ranks candidates by how often they appeared in
the same data access request as the source study. Only submitted,
non-archived, non-progress-report DARs score, and the reasons are worth
checking: a draft cart is not a request, an archived DAR should stop
counting, and a progress report carries its own reference_id, so counting
one would score its parent DAR more than once.

Both queries restrict candidates to public_visibility = TRUE, so a
recommendation never leaks the existence of a private study, and the source
study goes through verifyStudyVisibilityAccess.

No migration, no change to any existing response.

Depends on: branch 4 (extends MetricsService / MetricsResource).

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Where this sits in the DT-4061 stack

otchet-dt-3990-study-ratings-pi-details was too large to review meaningfully
(95 files, +8168), so it was split into eight PRs. This PR targets otchet-dt-4061-study-dar-metrics,
not develop, so its diff shows only its own work.

#	Branch	Base
1	otchet-dt-4061-study-visibility-authz	develop
1b	otchet-dt-4061-study-patch-ownership	otchet-dt-4061-study-visibility-authz
2	otchet-dt-4061-study-pi-details	otchet-dt-4061-study-visibility-authz
3	otchet-dt-4061-study-comments	otchet-dt-4061-study-pi-details
4	otchet-dt-4061-study-dar-metrics	otchet-dt-4061-study-comments
5	otchet-dt-4061-study-recommendations ← this PR	otchet-dt-4061-study-dar-metrics
6	otchet-dt-4061-study-asset-fields	otchet-dt-4061-study-recommendations
7	otchet-dt-4061-study-asset-endpoints	otchet-dt-4061-study-asset-fields

Branch 1b is a sibling rather than a link in the chain: nothing depends on it,
so it can be held or dropped without blocking the others. Land the numbered
chain in order — merging out of order, or squash-merging, will require rebasing
the descendants.

The split is verified lossless: branch 7 plus 1b reproduces the original branch
exactly, apart from five scripts/verify-study-*.sh helper scripts that were
dropped at the author's request. ./mvnw test-compile passes on each branch
independently.

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