fix: bump dependencies to resolve security vulnerabilities

[neoneye]

Summary

Patch-level dependency bumps to resolve 28 of 30 open Dependabot alerts (the remaining 2 are gradio, skipped due to large version jump needing manual testing).

worker_plan/pyproject.toml (7 bumps, resolves 22 alerts):

Package	Old	New	Severity	Issue
nltk	3.9.2	3.9.3	Critical	Zip Slip arbitrary file overwrite
aiohttp	3.13.2	3.13.3	High + 7 lower	HTTP parser zip bomb
urllib3	2.6.2	2.6.3	High	Decompression-bomb bypass on redirects
python-multipart	0.0.21	0.0.22	High	Arbitrary file write
pillow	12.0.0	12.1.1	High	OOB write loading PSD images
pypdf	6.5.0	6.7.5	7x Medium	DoS / memory exhaustion
filelock	3.20.1	3.20.3	Medium	TOCTOU symlink

frontend_single_user/requirements.txt (1 bump, resolves 1 alert):

Package	Old	New	Severity	Issue
python-multipart	0.0.20	0.0.22	High	Arbitrary file write

mcp_cloud/requirements.txt + worker_plan_database/requirements.txt (2 bumps, resolve 2 alerts):

Package	Old	New	Severity	Issue
Flask	3.1.1	3.1.3	Low	Missing Vary: Cookie header

Not addressed (2 remaining alerts): gradio 6.0.2 → 6.7.0 (SSRF + path traversal) — skipped due to major version gap requiring manual testing.

Test plan

• pip install succeeds with the updated versions
• Worker plan pipeline runs end-to-end
• Frontend single-user starts without import errors
• Verify Dependabot alerts auto-close after merge

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