Add MVP frontend scaffold with FastAPI + HTMX + TailwindCSS

- Created complete frontend structure with Jinja2 templates
- Implemented three main pages: Dashboard, Fleet Roster, and Unit Detail
- Added HTMX auto-refresh for real-time updates (10s interval)
- Integrated dark/light mode toggle with localStorage persistence
- Built responsive card-based UI with sidebar navigation
- Created API endpoints for status snapshot, roster, unit details, and photos
- Added mock data service for development (emit_status_snapshot)
- Implemented tabbed interface on unit detail page (Photos, Map, History)
- Integrated Leaflet maps for unit location visualization
- Configured static file serving and photo management
- Updated requirements.txt with Jinja2 and aiofiles
- Reorganized backend structure into routers and services
- Added comprehensive FRONTEND_README.md documentation

Frontend features:
- Auto-refreshing dashboard with fleet summary and alerts
- Sortable fleet roster table (prioritizes Missing > Pending > OK)
- Unit detail view with status, deployment info, and notes
- Photo gallery with thumbnail navigation
- Interactive maps showing unit coordinates
- Consistent styling with brand colors (orange, navy, burgundy)

Ready for integration with real Series3 emitter data.

backend/models.py

@@ -0,0 +1,18 @@
+from sqlalchemy import Column, String, DateTime
+from datetime import datetime
+from backend.database import Base
+
+
+class Emitter(Base):
+    """Emitter model representing a seismograph unit in the fleet"""
+    __tablename__ = "emitters"
+
+    id = Column(String, primary_key=True, index=True)
+    unit_type = Column(String, nullable=False)
+    last_seen = Column(DateTime, default=datetime.utcnow)
+    last_file = Column(String, nullable=False)
+    status = Column(String, nullable=False)  # OK, Pending, Missing
+    notes = Column(String, nullable=True)
+
+    def __repr__(self):
+        return f"<Emitter(id={self.id}, type={self.unit_type}, status={self.status})>"