Fix remaining code-review findings (15 bugs; #11 skipped as AID-compatible)

Backend:
- provider: fall back to parsing a plain JSON body when a server ignores
  stream=true (was: silent empty turn); error if response has no text (#5)
- memorybank: clamp cursors after undo/retry shrinks the action list, and
  translate summary_cursor (list position) to an Action.index boundary
  before comparing with Memory.source_end (#7)
- memorybank: pinned memories now count toward the top_k budget (#8)
- memorybank: cosine() returns 0.0 on dimension mismatch; changing the
  embedding model clears stored vectors so they re-embed (#9)
- scripting: MAX_STORY_CARDS cap now counts cards inserted during the
  hook, so a script can't add unbounded cards in one turn (#10)
- settings: /test tolerates non-dict JSON from /models (#13)
- scenarios: import accepts worldInformation as a story-card source (#14)

Frontend:
- per-key debounce timers in PlotPanel and ScenarioEditor — editing two
  things within 600ms no longer drops the first save (#15, #16)
- Continue button no longer discards typed input (#17)
- failed retry resyncs actions from the server instead of leaving the
  removed action missing (#18)
- Settings save/test surface errors instead of hanging on Testing… (#19)
- InsightsPanel ignores stale responses from superseded requests (#20)
- placeholder scan includes story-card trigger keys (#21)

addStoryCard returning the 0-based index (falsy for the first card) matches
real AI Dungeon per the scripting guidebook — kept, documented (#11).
Statuses updated in CODE_REVIEW_FINDINGS.md; stale entries for previously
fixed items (#1-4, #6, #12) corrected.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KFsGHju9szibJJa2YJcdbg

frontend/src/pages/Scenarios.jsx

@@ -49,7 +49,8 @@ export default function Scenarios() {
     const scenario = await api.getScenario(scenarioId)
     const names = extractPlaceholders(
       scenario.prompt, scenario.memory, scenario.authors_note, scenario.ai_instructions,
-      ...scenario.story_cards.map((c) => c.entry),
+      // Cards can carry ${placeholders} in trigger keys too, not just entries.
+      ...scenario.story_cards.flatMap((c) => [c.keys, c.entry]),
     )
     if (names.length === 0) return begin(scenarioId)
     setPending({ scenario, names })