Fix caller lineup never loading and add Wellspring cult callers

The lineup fix and the new characters both land in main.py, so they share a
commit rather than being split artificially.

Lineup fix — populate_backgrounds() was reachable only through
POST /api/session/reset, so taking calls without hitting reset left
session.caller_backgrounds empty for a whole show. Every caller got a hollow
prompt, collapsed into generic relationship filler, and Silas lost his lore.
Now start_call populates on demand, get_caller_prompt raises
EmptyCallerBackgroundError instead of emitting an empty prompt, /api/callers
reports lineup readiness to a header badge, and cross-episode topic dedup
widened from 2 shows to 10.

Wellspring callers — Doyle as the anchor defector plus a six-member pool,
grouped by new frontmatter fields (group, group_lead, group_weight, register,
explicitness). A faction is capped at one caller per show, with a 15% roll for
two pairing the lead with one other member. group_weight lets an anchor carrying
an arc get slots faster than texture characters.

Also detects model refusals, which arrive as ordinary 200s and previously
reached air as broken-character text or dead silence.

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

backend/services/audio.py

@@ -72,7 +72,7 @@ class AudioService:
         self.ident_channel: int = 15   # Channel for idents (stereo: ch 15+16)
         self.monitor_device: Optional[int] = 14  # Babyface Pro (headphone monitoring)
         self.monitor_channel: int = 1  # Channel for mic monitoring on monitor device
-        self.phone_filter: bool = False  # Phone filter on caller voices
+        self.phone_filter: bool = False  # Phone filter on caller voices — applied in post, not baked in
 
         # Ad playback state
         self._ad_stream: Optional[sd.OutputStream] = None