Add backend-agnostic self-update with uv + pipx support

[Copilot]

This change makes uv a first-class managed install backend alongside pipx, and removes installer-specific logic from self-update/version gating paths. self-update now detects and updates all managed PlexTraktSync installs across supported backends, including PR flows.

• Install backend abstraction

  • Introduced a generic managed-install model and backend interface in plextraktsync/util/packaging.py.
  • Added backend implementations for:
    • pipx (pipx list --json detection, upgrade/install PR commands)
    • uv (uv tool list detection, upgrade/install PR commands)
  • Added shared helpers for cross-backend discovery and self-update eligibility.

• Self-update orchestration

  • Refactored plextraktsync/commands/self_update.py to:
    • discover installs by backend
    • run backend-specific update commands per detected install
    • apply PR updates per backend
  • Kept pipx PR reinstall behavior scoped to the pipx backend only.

• CLI/Factory/Version generalization

  • Updated self-update command help text to describe both pipx and uv.
  • Changed Factory.enable_self_update to use backend-agnostic managed-install detection.
  • Replaced pipx-specific version-install check with a generic managed-install check while preserving PR/dev formatting behavior.

• Docs + tests

  • Updated README install/self-update docs to include uv and multi-backend update behavior.
  • Updated CONTRIBUTING PR-install workflow with uv guidance.
  • Added focused unit tests for backend detection, self-update gating, latest/PR update orchestration, pipx PR workaround, and partial backend availability.

# self-update now targets all detected managed installs, not only pipx
for install in list_managed_installs():
    backend = backend_for_name(install.backend)
    execp(backend.latest_update_command(install))

[Copilot]

Pull request overview

This PR refactors PlexTraktSync’s self-update flow to be install-backend agnostic by introducing a managed-install abstraction with backend implementations for pipx and uv, then updating CLI/docs/tests accordingly.

Changes:

• Added a backend interface + ManagedInstall model with discovery/update commands for pipx and uv.
• Refactored self-update to discover all managed installs and run backend-specific latest/PR update commands.
• Updated docs and added unit tests to cover backend detection and orchestration.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
plextraktsync/util/packaging.py	Adds backend abstraction, pipx/uv discovery, and backend-specific update command generation.
plextraktsync/commands/self_update.py	Orchestrates self-update across discovered managed installs/backends (latest + PR flows).
plextraktsync/factory/Factory.py	Enables the self-update command based on backend-agnostic “managed install exists” detection.
plextraktsync/util/Version.py	Generalizes “managed install” detection for PR/dev version formatting.
plextraktsync/cli.py	Updates self-update help text/examples to mention both backends.
README.md	Documents uv install/upgrade and multi-backend update behavior; updates command list description.
CONTRIBUTING.md	Documents PR install workflow for both pipx and uv (including uninstall guidance).
tests/test_self_update_packaging.py	Adds unit tests for backend detection and update orchestration behaviors.

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