Add markdown editor toolbar and data year display on import page

CMS editor: formatting toolbar with H1-H3, bold, italic, strikethrough,
links, images, lists, blockquotes, code blocks, horizontal rules.
Keyboard shortcuts (Ctrl+B/I/D/K). Improved markdown preview rendering.

Import page: shows current data summary with year badge, stats grid,
last import date. Year input for new imports. Preview with sample table.

Backend: added data_year and data_imported_at fields to TariffParams,
returned in stats endpoint. Import sets data_imported_at automatically.
Seed sets data_year=2018.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/schemas/schemas.py

@@ -56,6 +56,7 @@ class TariffParamsUpdate(BaseModel):
     recettes: float | None = None
     pmax: float | None = None
     vmax: float | None = None
+    data_year: int | None = None
 
 
 class TariffParamsOut(BaseModel):
@@ -64,6 +65,8 @@ class TariffParamsOut(BaseModel):
     recettes: float
     pmax: float
     vmax: float
+    data_year: int | None = None
+    data_imported_at: datetime | None = None
 
     model_config = {"from_attributes": True}
 
@@ -91,6 +94,8 @@ class HouseholdStats(BaseModel):
     avg_volume: float
     median_volume: float
     voted_count: int
+    data_year: int | None = None
+    data_imported_at: datetime | None = None
 
 
 class ImportPreview(BaseModel):