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_caller_prompt.py

@@ -1,4 +1,25 @@
-from backend.main import get_caller_prompt
+import pytest
+
+from backend.main import EmptyCallerBackgroundError, get_caller_prompt
+
+
+def test_empty_background_raises_instead_of_hollow_prompt():
+    """A hollow prompt still generates dialog — the model invents generic
+    relationship filler and regulars lose their lore. Fail loudly instead."""
+    with pytest.raises(EmptyCallerBackgroundError):
+        get_caller_prompt({})
+
+
+def test_background_missing_every_identity_field_raises():
+    caller = {"voice": "Sebastian", "age": 52, "location": "Terlingua"}
+    with pytest.raises(EmptyCallerBackgroundError):
+        get_caller_prompt(caller)
+
+
+def test_partial_background_still_builds():
+    """A name alone is enough to reach the regulars lookup, so don't block it."""
+    prompt = get_caller_prompt({"name": "Silas"})
+    assert "Silas" in prompt
 
 
 def test_prompt_includes_identity_and_situation():