Add three Fivetran interview practice problems

[tomtoto757]

Summary

Add three Fivetran practice problems: Unique Near-Equal Target-Sum Pairs, Connected Groups, and Aladdin and the Magic Carpet.

Resolve conflicts against current master while preserving all 2,182 existing README rows. The README changes only by the three additions and Coding count (1,796 → 1,799). Regenerate stale Coding, SQL, System design, and Low-level design pages from the resulting README; AI coding was already current. No source identity or source-platform URL was added.

Verification

• PYTHONDONTWRITEBYTECODE=1 python3 -m unittest scripts/test_sync_practice_formats.py: 11 passed.
• python3 scripts/refresh-freshness.py: valid dates, row formatting, descending order, and size check passed.
• python3 scripts/sync-practice-formats.py --catalog-file /tmp/tech-oa-pr163-catalog.json --check: passed. This scoped temporary snapshot uses current public coding metadata and master-pinned design metadata, preserving the existing catalog scope.
• All five generated format pages exactly reproduce ordered README projections; counts, unique routes, and matching question/Practice URLs verified.
• All three added FastPrep routes returned HTTP 200.
• git diff --check origin/master...HEAD: passed; worktree clean.
• GitHub reports CLEAN/MERGEABLE for the reviewed head; no PR status checks are configured or attached. Type checking and application builds are not applicable to this Markdown-only diff.

Independent regression review

• Reviewed final head 4c84c0c01d1a17925f293ff5881ab973a2db99a4 against latest master 30288b16b71ddb0899f10f04d96ec97a144577a7.
• Separate reviewer inspected the complete diff, original PR additions, all master rows, generated-page consumers, company list, route uniqueness, link matching, and existing numeric-route inventory; independently ran the 11 tests, scoped generator check, and diff check.
• Findings: no introduced regressions. All 2,182 master rows and the original three Fivetran additions are preserved. Company list and 61 pre-existing numeric routes are unchanged.
• Remaining pre-existing limitation: using identical current live catalog input, full sync generates 500,117 README bytes on master and 500,810 on this head, exceeding the existing 500,000-byte ceiling in both cases. The scoped committed README is 498,926 bytes. Seven unrelated new design entries remain outside this conflict-resolution scope; the hourly full-sync size issue requires separate maintenance.