Add rename_recording endpoint with path validation and collision detection

[Copilot]

Adds backend support for renaming recordings, exposing a POST /rename_recording endpoint for the capture app workflow.

Changes

• recording_db.py — rename_recording_entry()

  • Rejects absolute paths and .. traversal segments in new_filename
  • Enforces that only the basename may change (parent directory must remain the same)
  • Checks for DB collisions (Recording with matching filename) and filesystem collisions (os.path.exists) before proceeding; raises FileExistsError on conflict
  • Renames the directory on disk if it exists, then updates the DB

• fastapi.py — POST /rename_recording

  • Accepts RenameRecordingData (participant, filename, new_filename)
  • Maps FileExistsError → HTTP 409 Conflict, ValueError → HTTP 404

Example

# Accepted: same directory, new basename
rename_recording_entry(db, "p123", "/data/p123/trial_20250101_120000", "/data/p123/trial_20250101_renamed")

# Rejected: path traversal
rename_recording_entry(db, "p123", "/data/p123/trial_A", "/data/p123/../p999/trial_A")
# ValueError: new_filename must not contain '..' path segments

# Rejected: directory change
rename_recording_entry(db, "p123", "/data/p123/trial_A", "/data/p999/trial_A")
# ValueError: new_filename must be in the same directory as the original recording

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