feat: persist interview prep instead of regenerating on every open

Interview prep re-ran its AI call every time the tab opened -- flagged
in the product teardown as work evaporating on every re-open (cost,
latency, and non-determinism for no reason). GetInterviewPrep now
persists one note per job application and reuses it on subsequent
reads, only regenerating when the selected attachment context changes
or a refresh is explicitly requested.

- InterviewPrepNote: one row per (owner, job), keyed additionally by
  an attachment-selection fingerprint so picking different attachments
  correctly triggers a fresh brief without needing an explicit flag.
- GetInterviewPrep gained a `refresh` query param; the frontend adds a
  small "Regenerate" button as the explicit escape hatch for when the
  underlying job/notes have changed since the note was written.
- Both SQLite (dev) and MySQL/MariaDB (prod) reconciler dialects.
- 3 new tests: reuse across calls, refresh regenerates, attachment
  context change regenerates. Verified against the real dev DB.

job-tracker-ui/src/components/JobDetailsDialog.tsx

@@ -1,4 +1,4 @@
-import React, { useEffect, useMemo, useState } from "react";
+import React, { useCallback, useEffect, useMemo, useState } from "react";
 
 import {
   Box,
@@ -364,6 +364,19 @@ export default function JobDetailsDialog({ open, jobId, onClose, initialTab = 0,
     }).catch(() => setInterviewPrep(null)).finally(() => setLoadingInterviewPrep(false));
   }, [open, jobId, tab, interviewPrep, selectedAttachmentCsv, interviewPrepCache]);
 
+  // Interview prep is now persisted server-side (career-workspace-implementation-roadmap.md
+  // Phase F5) so it survives tab switches without re-running the AI call. Regenerate is the
+  // explicit escape hatch for when the underlying job/notes have changed since it was written.
+  const regenerateInterviewPrep = useCallback(() => {
+    if (!jobId) return;
+    setLoadingInterviewPrep(true);
+    api.get<InterviewPrepResponse>(`/jobapplications/${jobId}/interview-prep`, { params: { attachmentIds: selectedAttachmentCsv || undefined, refresh: true } }).then((r) => {
+      interviewPrepCache.setCached(`${jobId}:interview-prep:${selectedAttachmentCsv || "none"}`, r.data);
+      setInterviewPrep(r.data);
+      toast("Interview prep regenerated.", "success");
+    }).catch((error: any) => toast(getApiErrorMessage(error, "Failed to regenerate interview prep."), "error")).finally(() => setLoadingInterviewPrep(false));
+  }, [jobId, selectedAttachmentCsv, interviewPrepCache, toast]);
+
   useEffect(() => {
     setFollowUpDraft(null);
     setCandidateFit(null);
@@ -1171,6 +1184,11 @@ export default function JobDetailsDialog({ open, jobId, onClose, initialTab = 0,
 
         {tab === 7 && (
           <Box>
+            <Box sx={{ display: "flex", justifyContent: "flex-end", mb: 1.5 }}>
+              <Button size="small" variant="outlined" disabled={loadingInterviewPrep} onClick={regenerateInterviewPrep}>
+                {loadingInterviewPrep ? "Regenerating..." : "Regenerate"}
+              </Button>
+            </Box>
             {loadingInterviewPrep ? <Box sx={{ py: 4, display: "flex", justifyContent: "center" }}><CircularProgress size={28} /></Box> : interviewPrep ? (
               <Box sx={{ display: "flex", flexDirection: "column", gap: 2 }}>
                 <DraftCard title={t("jobDetailsInterviewPrepBrief")} content={interviewPrep.summary} />