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>

frontend/css/style.css

@@ -246,6 +246,25 @@ header button:hover, .header-link-btn:hover {
     font-weight: normal;
 }
 
+.lineup-status {
+    font-size: 0.7rem;
+    font-weight: 600;
+    margin-left: 0.5rem;
+    padding: 0.1rem 0.4rem;
+    border-radius: 3px;
+    vertical-align: middle;
+}
+
+.lineup-status.ready {
+    color: #4ade80;
+    background: rgba(74, 222, 128, 0.12);
+}
+
+.lineup-status.empty {
+    color: #f87171;
+    background: rgba(248, 113, 113, 0.15);
+}
+
 details.caller-background {
     font-size: 0.85rem;
     color: var(--text-muted);