Stop Whisper misspelling the intern's name as Devin

The intern reached 21 of 58 published transcripts as "Devin". The obvious fix —
seed his name in the Whisper initial prompt — only works for one of the two
transcription paths:

- backend/services/transcription.py (live show) uses mlx_whisper and does take
  an initial_prompt. Added Devon there. This also picks up the Big Bend prompt
  rewrite from the relocation work, which was sitting uncommitted.
- publish_episode.py, which actually produces the published transcripts, uses
  LightningWhisperMLX, whose transcribe() signature is (audio_path, language).
  It accepts no initial_prompt at all, so there is nothing to seed.

So the published path gets a deterministic correction pass instead:
fix_proper_nouns() rewrites known mishearings after transcription, preserving
casing (Devin/DEVIN/devin -> Devon/DEVON/devon) and matching whole words only,
so "Devinshire" is left alone. Verified against the real episode 58 transcript:
24 occurrences to 0, output byte-identical to the manual relabel in bf1afef.

Swapping the publish path to mlx_whisper would allow a real prompt, but that
changes the transcription engine for every episode and is a bigger call than
this warrants.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/services/transcription.py

@@ -103,7 +103,7 @@ async def transcribe_audio(audio_data: bytes, source_sample_rate: int = None,
         audio_16k = audio
 
     # Build initial prompt — context helps Whisper with names and topic-specific words
-    initial_prompt = "Luke at the Roost, a late-night radio talk show in New Mexico. The host Luke talks to callers about life, relationships, sports, politics, and pop culture."
+    initial_prompt = "Luke at the Roost, a late-night radio talk show in Alpine, Texas, in the Big Bend region. The host Luke talks to callers about life, relationships, sports, politics, and pop culture, with his intern Devon. Callers reference Alpine, Marfa, Marathon, Terlingua, Fort Stockton, and Big Bend."
     if context_hint:
         initial_prompt += f" {context_hint}"