feat: integrate Career Workspace foundation from feature/career-workspace

Recover the F1 Career Profile foundation + AI-workspace persistence from the
unmerged feature/career-workspace branch, so Phase 2 builds on the documented,
tested target state instead of re-deriving it. Foundation only — CV Builder
commits (variants, ATS badge, rewrite diff) stay deferred per "do not build CV
Builder yet". See docs/career-workspace-branch-assessment.md.

Squashed from 3 branch commits (235e291, 5916f09, 00a035e), resolved against
main + Phase 0:

- CareerProfile + CareerProfileVersion (append-only history), dual-written from
  every profile save path via CareerProfileService. ApplicationUser.
  ProfileCvStructureJson stays authoritative; the tables mirror it. Stable item
  IDs assigned to jobs/education/certifications/projects (the prerequisite for
  future variant lineage). CvDateNormalizer for free-text -> YYYY-MM.
- InterviewPrepNote + AiWorkspaceNote: cache AI interview prep / candidate fit /
  focus plan keyed by an attachment-context signature, so they stop regenerating
  (and re-spending the provider) on every open.

Conflict resolutions (union, favouring current code + Phase 0):
- JobTrackerContext / StartupInitializationExtensions: kept Phase 0's tables and
  reconciler blocks, added the career/interview/ai-note tables (both SQLite and
  MySQL dialects).
- ProfileCvController: dropped the branch's in-file DTO records (main defines them
  in ProfileCvDtos.cs) and the LayoutFamily/AtsRating template fields (deferred
  ATS-badge work), keeping main's 7-arg CvTemplateDescriptor.
- JobApplicationsController: kept the branch's cache-check, restored main's
  AsNoTracking on the read-only user load.

Tables ship empty (verified dev); nothing to migrate.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Models/AiWorkspaceNote.cs

@@ -0,0 +1,19 @@
+namespace JobTrackerApi.Models;
+
+// Generic persistence for per-job AI workspace outputs whose shape is irregular/nested enough
+// that per-field columns (as used by InterviewPrepNote) would be unreasonable -- candidate fit
+// and focus plan each make several AI calls and return DTOs with 6-13 fields including nested
+// objects. One row per (owner, job, note type); ResultJson is the serialized DTO. See
+// career-workspace-implementation-roadmap.md Phase F5 -- "persist interview prep / fit outputs".
+public sealed class AiWorkspaceNote
+{
+    public int Id { get; set; }
+    public string OwnerUserId { get; set; } = string.Empty;
+    public int JobApplicationId { get; set; }
+    public JobApplication? JobApplication { get; set; }
+    // "candidate-fit" | "focus-plan"
+    public string NoteType { get; set; } = string.Empty;
+    public string AttachmentContextSignature { get; set; } = string.Empty;
+    public string ResultJson { get; set; } = string.Empty;
+    public DateTimeOffset GeneratedAtUtc { get; set; } = DateTimeOffset.UtcNow;
+}