feat(search): add relevance-based ranking for campaign and donation s…

[DingiDinigi]

Closes #148

Summary

Adds relevance-based ranking for campaign and donation search, while keeping existing pagination, filtering, and non-relevance sort options fully untouched.

Changes

• search.service.ts: When sortBy: 'relevance' is requested, searchCampaigns/searchDonations route to new searchCampaignsByRelevance/searchDonationsByRelevance methods. These compute a per-field weighted relevance score via a parameterized $queryRaw CTE (exact match > prefix match > contains match tiers, weighted per field), order by score DESC, id ASC for deterministic results, and paginate with LIMIT/OFFSET in the same query. Ranked ids are then hydrated via a normal findMany (same include shape as today) and re-sorted in JS to preserve rank order, since WHERE id IN (...) doesn't guarantee row order.
• All other sort options continue through the original, unmodified orderBy path.
• User query input is LIKE-escaped before use in the raw SQL (fixes a minor pre-existing gap where special characters weren't escaped in the contains path either).
• searchCampaigns's existing response cache covers the relevance path automatically (same cache-key strategy, no changes needed there).

Testing

• Added 8 new tests covering: relevance path is selected correctly (and never touches the old count/orderBy path), score-descending/id-tiebreak ordering survives hydration reordering, pagination params (LIMIT/OFFSET) are correctly computed and passed, and non-relevance sorts are unaffected (regression).
• Fixed a pre-existing test-environment gap: searchCampaigns tests previously hung indefinitely (10s timeout) because the test file didn't mock the Redis-backed cache utility, causing ioredis to retry a real connection forever in this sandbox. Added a mock so the cache always misses in tests — this was necessary to actually exercise searchCampaigns (relevance or otherwise) in this environment, not scope creep.
• Full suite: 21/21 passing. npx tsc --noEmit: zero new type errors.