refactor: PDF filenames with_skills → with-skills (cosmetic only)

Changed PDF filename format to use hyphens instead of underscores for
consistency with other filename components, while keeping API parameter
as `version=with_skills`.

## Changes

**Backend:**
- internal/handlers/cv.go: Add underscore-to-hyphen conversion in filename generation
  - New logic: `strings.ReplaceAll(version, "_", "-")` for filename only
  - API parameter unchanged: still accepts `version=with_skills`

**Tests:**
- internal/handlers/pdf_test.go: Update expected filenames to use hyphens
  - cv-*-with_skills-*.pdf → cv-*-with-skills-*.pdf

**Documentation:**
- Updated all PDF filename references to use hyphens
- PDF-EXPORT-FEATURE.md
- doc/LONG-PDF-GENERATION.md
- PDF-VALIDATION-REPORT.md (new validation report)

**PDFs:**
- Regenerated all 8 PDFs with new naming convention
- Old: cv-short-with_skills-jamr-2025-en.pdf
- New: cv-short-with-skills-jamr-2025-en.pdf

## Examples

API calls unchanged:
- GET /export/pdf?version=with_skills (still works)

Generated filenames:
- cv-short-with-skills-jamr-2025-en.pdf ✓
- cv-long-with-skills-jamr-2025-es.pdf ✓

**Tests:** All passing ✓
**API:** Backwards compatible ✓

internal/handlers/pdf_test.go

@@ -178,12 +178,12 @@ func TestExportPDF_FilenameGeneration(t *testing.T) {
 				"length":  "long",
 				"version": "with_skills",
 			},
-			expectedFilename: "CV-Juan-Andrés-Moreno-Rubio-es-long-with_skills.pdf",
+			expectedFilename: "CV-Juan-Andrés-Moreno-Rubio-es-long-with-skills.pdf",
 		},
 		{
 			name:             "Defaults (en, short, with_skills)",
 			params:           map[string]string{},
-			expectedFilename: "CV-Juan-Andrés-Moreno-Rubio-en-short-with_skills.pdf",
+			expectedFilename: "CV-Juan-Andrés-Moreno-Rubio-en-short-with-skills.pdf",
 		},
 	}