feat: centralize media caching

[Teagan42]

What

• add MediaCache helper for payload, poster, and background data
• attach MediaCache to PlexServer and update server tools to use it
• verify cache usage in server tests and bump version to 0.26.11

Why

• simplifies cache management and removes scattered cache globals

Affects

• server caching logic
• test expectations of cache location

Testing

• uv run ruff check .
• uv run pytest

Documentation

• N/A

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https://chatgpt.com/codex/tasks/task_e_68c64c3787208328918d38a322c23dc1

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
mcp_plex
cache.py	36	4	89%	22, 50–52
loader.py	197	10	95%	32–34, 331, 334–338, 663
server.py	294	20	93%	95–96, 218, 446, 450, 472–478, 510, 520, 551–552, 569–570, 593, 596–597, 656
TOTAL	631	34	95%

Tests	Skipped	Failures	Errors	Time
29	0 💤	0 ❌	0 🔥	33.766s ⏱️