feat(s3): add short access links

[xqvvu]

Summary

• Add short S3 download alias URLs (/api/system/file/d/<alias>.<exp>.<sig>) backed by stable Mongo alias documents.
• Add short upload session URLs (/api/system/file/u/<token>) backed by hashed Mongo session records.
• Reuse shared S3 proxy handlers for new short routes and legacy JWT compatibility routes.
• Update dataset/search/quote preview URL generation to use async shortlink signing.
• Add PR1 design notes and focused shortlink/proxy compatibility tests.

Tests

• git diff --check
• From packages/service: pnpm exec vitest run -c vitest.config.ts test/common/s3/accessLink.test.ts test/common/s3/token.test.ts test/core/dataset/utils.test.ts test/core/dataset/data/controller.test.ts
• From projects/app: pnpm exec vitest run -c vitest.config.ts test/api/system/file/accessLink.test.ts test/api/system/file/sourceContentType.test.ts

Notes

• Legacy JWT download/upload routes are kept for already-issued links, but new proxy signing paths now issue short access links.
• PR2 upload policy validation and PR3 ChatBox upload abort remain out of scope for this PR.

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Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	16.43%	1354 / 8239
🔵	Statements	16.36%	1414 / 8642
🔵	Functions	14.45%	288 / 1993
🔵	Branches	13.31%	618 / 4642

File Coverage

No changed files found.

Generated in workflow #1267 for commit 1351e50 by the Vitest Coverage Report Action

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