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/models/models.py

@@ -47,6 +47,8 @@ class TariffParams(Base):
     recettes = Column(Float, default=75000.0)
     pmax = Column(Float, default=20.0)
     vmax = Column(Float, default=2100.0)
+    data_year = Column(Integer, nullable=True)
+    data_imported_at = Column(DateTime, nullable=True)
 
     commune = relationship("Commune", back_populates="tariff_params")