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>

tests/test_call_start_lineup_guard.py

@@ -0,0 +1,72 @@
+"""Regression guard: a show once ran with session.caller_backgrounds empty because
+populate_backgrounds() was reachable only via POST /api/session/reset. Every caller
+got a hollow prompt and collapsed into generic relationship filler.
+"""
+import asyncio
+
+import pytest
+
+import backend.main as m
+
+
+@pytest.fixture
+def quiet_call(monkeypatch):
+    monkeypatch.setattr(m.audio_service, "stop_caller_audio", lambda *a, **k: None)
+    monkeypatch.setattr(m, "_maybe_generate_callback", lambda: None)
+    monkeypatch.setattr(m, "_enrich_background_async", lambda key: asyncio.sleep(0))
+    monkeypatch.setattr(m.session, "intern_monitoring", False)
+    m.session.current_caller_key = None
+    yield
+    # asyncio.run() closes the loop it creates and clears the thread's current
+    # loop. tests/test_caller_service.py still calls asyncio.get_event_loop(),
+    # which then raises — and this module sorts ahead of it. Leave a usable loop.
+    asyncio.set_event_loop(asyncio.new_event_loop())
+
+
+def test_start_call_populates_empty_lineup(monkeypatch, quiet_call):
+    key = next(iter(m.CALLER_BASES))
+    m.session.caller_backgrounds = {}
+    called = []
+
+    async def fake_populate():
+        called.append(True)
+        m.session.caller_backgrounds = {
+            key: {"name": "Silas", "voice": "Sebastian", "identity": "commune founder",
+                  "situation": "the convoy stopped", "specific_details": []}
+        }
+
+    monkeypatch.setattr(m.session, "populate_backgrounds", fake_populate)
+    asyncio.run(m.start_call(key))
+
+    assert called, "start_call must populate backgrounds when the lineup is empty"
+    assert m.session.caller_backgrounds, "lineup should be populated after the call starts"
+
+
+def test_start_call_does_not_repopulate_existing_lineup(monkeypatch, quiet_call):
+    key = next(iter(m.CALLER_BASES))
+    m.session.caller_backgrounds = {
+        key: {"name": "Silas", "voice": "Sebastian", "identity": "commune founder",
+              "situation": "the convoy stopped", "specific_details": []}
+    }
+    called = []
+
+    async def fake_populate():
+        called.append(True)
+
+    monkeypatch.setattr(m.session, "populate_backgrounds", fake_populate)
+    asyncio.run(m.start_call(key))
+
+    assert not called, "an existing lineup must not be regenerated mid-show"
+
+
+def test_prompt_from_populated_background_is_not_hollow():
+    prompt = m.get_caller_prompt({
+        "name": "Silas", "identity": "founder of The Wellspring",
+        "situation": "the convoy stopped on the shoulder of 118",
+        "reason_calling": "Priscilla will not come out of the trailer",
+        "secret_want": "to hear he is not the reason she broke",
+        "specific_details": ["livestock trailer", "nine years in"],
+    })
+    assert "Silas" in prompt
+    assert "Wellspring" in prompt
+    assert "You are . " not in prompt