Allow user to provide a theme for audio lesson generation

[mircealungu]

Idea

Let the user optionally provide a theme (a short phrase) when generating a daily audio lesson. The theme would be passed to the LLM prompt so it tries to set the dialogues in that context.

For example, a user learning Dutch could type "at the supermarket" and the generated lesson dialogues would use supermarket-related scenarios for the vocabulary words.

Implementation

Changes span both API and frontend (~20 lines of logic total):

API (this repo)

1. Prompt template (audio_lessons/prompts/meaning_lesson--...v2.txt): Add a {theme} placeholder near the "brief scenario" instruction (line 12)
2. Script generator (audio_lessons/script_generator.py): Add theme=None param to generate_lesson_script(), pass to template
3. Generation pipeline: Thread theme through generate_daily_lesson endpoint → preparation dict → background thread → build_daily_lesson() → generate_lesson_script()

Frontend (zeeguu/web)

4. TodayAudio.js: Add an optional text input on the generate screen, send as form data

Notes

• Theme should be optional — if not provided, behavior is unchanged
• The prompt should treat the theme as a suggestion, not a hard requirement (some words may not fit the theme naturally)