Add CE1 board 3 and CE2 boards 2 and 3 with new backdrops (0.7.0)

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

data/curriculum/ce1_verbes_present.json

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+{
+  "level": "CE1",
+  "region_id": "montagne_des_verbes",
+  "chapter_id": "verbes_present",
+  "title": "Les verbes au présent",
+  "region_title": "La montagne des verbes",
+  "order": 3,
+  "backdrop": "mountain",
+  "rule_ids": [
+    "regle_verbe_etre_present",
+    "regle_verbe_avoir_present",
+    "regle_verbe_er_present"
+  ],
+  "review_status": "draft"
+}

data/curriculum/ce2_lettres_et_mots.json

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+{
+  "level": "CE2",
+  "region_id": "ile_des_lettres",
+  "chapter_id": "lettres_et_mots",
+  "title": "Les lettres et les mots",
+  "region_title": "L'île des lettres",
+  "order": 3,
+  "backdrop": "island",
+  "rule_ids": [
+    "regle_feminin_noms_adjectifs",
+    "regle_g_ge_gu",
+    "regle_lettre_muette_finale"
+  ],
+  "review_status": "draft"
+}

data/curriculum/ce2_verbes_accords.json

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+{
+  "level": "CE2",
+  "region_id": "desert_des_verbes",
+  "chapter_id": "verbes_accords",
+  "title": "Les verbes : accords et formes",
+  "region_title": "Le désert des verbes",
+  "order": 2,
+  "backdrop": "desert",
+  "rule_ids": [
+    "regle_accord_sujet_verbe",
+    "regle_e_er_infinitif",
+    "regle_futur_simple"
+  ],
+  "review_status": "draft"
+}