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/regulars_v2.py

@@ -24,6 +24,12 @@ class Regular:
     arc_state: str
     lore_body: str
     file_path: Path
+    group: str = ""          # Faction key. Members of a group are capped per show.
+    group_lead: bool = False  # On a two-up night, the lead is preferred + one other
+    group_weight: float = 1.0  # Relative odds of taking the group's slot. Anchors
+                               # carrying an arc need slots faster than texture does.
+    register: str = ""       # How this character plays: dry, evangelical, exhausted...
+    explicitness: str = "low"  # low | medium | high — how frankly they discuss sex
 
 
 def load_regular(path: Path) -> Regular:
@@ -37,6 +43,15 @@ def load_regular(path: Path) -> Regular:
         if ":" in line:
             k, v = line.split(":", 1)
             fm[k.strip()] = v.strip()
+    explicitness = fm.get("explicitness", "low").strip().lower()
+    if explicitness not in ("low", "medium", "high"):
+        explicitness = "low"
+    try:
+        group_weight = float(fm.get("group_weight", 1.0))
+    except (TypeError, ValueError):
+        group_weight = 1.0
+    if group_weight <= 0:
+        group_weight = 1.0
     return Regular(
         name=fm["name"],
         voice=fm["voice"],
@@ -44,6 +59,11 @@ def load_regular(path: Path) -> Regular:
         arc_state=fm.get("arc_state", ""),
         lore_body=body,
         file_path=path,
+        group=fm.get("group", "").strip(),
+        group_lead=fm.get("group_lead", "").strip().lower() in ("true", "yes", "1"),
+        group_weight=group_weight,
+        register=fm.get("register", "").strip(),
+        explicitness=explicitness,
     )