User-entered metadata immediately overwritten by extraction fallback

[winterbourne]

Symptoms

• Fill in Title, Author, and other metadata in the "Book Metadata" section of the wizard
• Click "Continue" or navigate away
• Values disappear and are replaced with an alphanumeric extraction ID (e.g., 423d828962c34d2b8a53bbe91176305a)
• The title intro (if enabled) speaks this hash as individual letters instead of the title you entered
• Happens on every step navigation and job creation

Root Cause

• build_pending_job_from_extraction() in form.py extracts metadata from the uploaded file
• If no metadata is embedded in the file, it falls back to the filename/extraction ID
• This extraction-based metadata is set after apply_book_step_form() tries to save user input
• User-entered values are overwritten by the extraction fallback

Expected Behavior

• User-entered metadata should take precedence over extracted or auto-generated fallbacks
• Metadata should persist throughout the wizard and into the final job
• Job details should always show the user's title/author, not the extraction hash

Impact

• CRITICAL – Users cannot customize audiobook metadata
• No workaround (metadata is wiped on every interaction)
• Output files have meaningless titles (hash-based)
• Title intro speaks gibberish instead of book name

Fix Required

1. Apply user form metadata after extraction, not before
2. Use extracted metadata only as a fallback if the user didn't enter anything
3. Preserve user-entered fields through all wizard steps and job creation