Fix case-insensitive clip matching and zero-match output

[LauraGPT]

Summary

• match ASCII letters case-insensitively while preserving character-to-timestamp indexes
• use the same matching behavior for audio and video clipping
• return no video output and emit a warning when destination text has no match
• cover uppercase recognition output, zero-match behavior, and existing duplicate matching

Verification

• python -m pytest -q tests/test_duplicate_text_matching.py (7 passed)
• python -m pytest -q tests in an isolated environment with the repository's optional providers (61 passed, 1 skipped)
• git diff --check

Closes #198

[gemini-code-assist]

Caution

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