fix: use UUID for podcast episode directory names

[lfnovo]

Summary

• Podcast episode names with spaces or special characters (e.g. My Episode: Part 1) were used directly as directory names, causing filesystem errors on Linux/Docker
• Replaced episode name with UUID for directory and file naming, eliminating all path safety issues
• Original episode name is preserved in the database for display purposes

Closes #663

Test plan

• Added 6 unit tests in tests/test_podcast_path.py covering UUID format, path validity, no collisions, no unsafe characters, POSIX compatibility, and real filesystem creation
• Manual test: create a podcast with spaces and special characters in the name and verify it completes successfully

[cubic-dev-ai]

1 issue found across 2 files

Confidence score: 3/5

• There is a meaningful regression-risk gap in tests/test_podcast_path.py: the test reimplements path generation logic instead of calling the real podcast path builder, so failures in production logic may be missed.
• Given the medium severity (6/10) and high confidence (9/10), this is more than a housekeeping issue—it weakens trust in test coverage for path handling behavior.
• This PR may still be mergeable, but it carries some risk until the test is aligned to exercise the actual implementation and uses problematic_names as intended.
• Pay close attention to tests/test_podcast_path.py - test logic may mask real path-builder breakages.

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="tests/test_podcast_path.py">

<violation number="1" location="tests/test_podcast_path.py:13">
P2: This test file reimplements path generation instead of invoking the actual podcast path builder, so it can pass even when production code is broken. In particular, `problematic_names` are never used to drive the generated path.</violation>
</file>

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