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/caller_gen.py

@@ -66,6 +66,14 @@ def resolve_voice(suggestion: str, roster: list[str]) -> str:
     return random.choice(roster)
 
 
+# Per-character dial rather than a global switch, so explicitness varies by who
+# called instead of the whole show moving together.
+_EXPLICITNESS_GUIDANCE = {
+    "low": "alludes to sex obliquely at most, deflects or changes the subject when it comes up — their discomfort is part of the characterization",
+    "medium": "references sex bluntly and matter-of-factly when it's relevant, but doesn't dwell or go into detail",
+    "high": "talks about sex and kink frankly, graphically, and without embarrassment — it is simply how this person talks",
+}
+
 BATCH_SYSTEM_PROMPT = """You are writing a roster of callers for Luke's late-night radio show, broadcast out of Alpine, Texas, in the Big Bend country of far West Texas.
 
 WHERE THESE CALLERS LIVE: The show's world is the Big Bend region of far West Texas — high desert, remote, dark skies, ranching country up against the Mexico border. Most callers are from or know this area. Ground them in REAL places and facts only — never invent businesses or landmarks that don't exist. The towns:
@@ -92,6 +100,8 @@ ANTI-COLLISION RULE — THIS IS NON-NEGOTIABLE: All callers in this roster must
 
 Do not default to sitcom plots. Real humans are specific and strange. Give each caller details that could only belong to them.
 
+ADULT SHOW: This is a late-night show for adults. Callers are adults and may talk about sex, relationships, and kink frankly and without euphemism when it fits who they are — this is not a family program and you should not sanitize a caller whose story is genuinely about sex. But do NOT make the roster horny: at most 1-2 of the 10 callers should have sex as their central subject, or the show flattens into one note. Everyone involved in any sexual material is a consenting adult. Where a situation involves pressure, manipulation, or coercion, write it with real weight — that is a harm, not a punchline, and the host should be able to react to it as one.
+
 OPENING LINE RULES: Each caller's opening_line must be unique and specific to their situation. NEVER write "I've been listening for X years" or "long-time listener, first-time caller" or any variant. The caller should jump into their story or problem immediately — nervous, excited, angry, whatever fits. The opening line is the hook that makes the audience lean in.
 
 You will output strict JSON with a "callers" array. Each caller has exactly these fields: name, age, voice_suggestion, location, identity, situation, reason_calling, opening_line, secret_want, specific_details (array of 2-3 strings), emotional_register."""
@@ -133,6 +143,9 @@ def build_batch_prompt(ctx: dict) -> str:
             lines.append(f"### {r['name']}")
             lines.append(r["lore"])
             lines.append(f"Current arc state: {r['arc_state']}")
+            if r.get("register"):
+                lines.append(f"How {r['name']} plays: {r['register']}")
+            lines.append(f"Explicitness for {r['name']}: {_EXPLICITNESS_GUIDANCE[r.get('explicitness', 'low')]}")
             lines.append("")
             lines.append(f"For {r['name']}: invent a fresh reason he is calling tonight — a new development, grievance, or specific recent event. DO NOT alter his voice, personality, or core traits. Write a new scene for an existing character.")
             lines.append(f"CRITICAL — this caller MUST appear in the output JSON with EXACTLY these fields locked: name=\"{r['name']}\", voice_suggestion=\"{r['voice']}\", age={r['age']}. Do NOT rename this caller. Do NOT give him a different voice. Do NOT change his age. Only invent: location, identity, situation, reason_calling, opening_line, secret_want, specific_details, emotional_register.")
@@ -191,6 +204,8 @@ async def generate_regular_situation(regular: dict, ctx: dict) -> dict:
         f"### {regular['name']}",
         regular["lore"],
         f"Current arc state: {regular['arc_state']}",
+        *([f"How {regular['name']} plays: {regular['register']}"] if regular.get("register") else []),
+        f"Explicitness for {regular['name']}: {_EXPLICITNESS_GUIDANCE[regular.get('explicitness', 'low')]}",
         "",
         f"Invent a fresh reason {regular['name']} is calling tonight — a new development, grievance, or specific recent event. DO NOT alter his voice, personality, or core traits.",
         "",