Add flight comparator web app with full scan pipeline

Full-stack flight price scanner built on fast-flights v3 (SOCS cookie bypass):

Backend (FastAPI + SQLite):
- REST API with rate limiting, Pydantic v2 validation, paginated responses
- Scan pipeline: resolves airports, queries every day in the window, saves
  individual flights + aggregate route stats to SQLite
- Background async scan processor with real-time progress tracking
- Airport search endpoint backed by OpenFlights dataset
- Daily scan window (all dates, not monthly samples)

Frontend (React 19 + TypeScript + Tailwind CSS v4):
- Dashboard with live scan status and recent scans
- Create scan form: country mode or specific airports (searchable dropdown)
- Scan detail page with expandable route rows showing individual flights
  (date, airline, departure, arrival, price) loaded on demand
- AirportSearch component with debounced live search and multi-select

Database:
- scans → routes → flights schema with FK cascade and auto-update triggers
- Migrations for schema evolution (relaxed country constraint)

Tests:
- 74 tests: unit + integration, isolated per-test SQLite DB
- Confirmed flight fixtures in tests/confirmed_flights.json (50 real flights,
  BDS→FMM Ryanair + BDS→DUS Eurowings, scraped Feb 2026)
- Integration tests parametrized from confirmed routes

Docker:
- Multi-stage builds, Compose orchestration, Nginx reverse proxy

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

flight-comparator/date_resolver.py

@@ -0,0 +1,132 @@
+"""
+Date resolution and seasonal scan logic for flight comparator.
+"""
+
+from datetime import date, timedelta
+from dateutil.relativedelta import relativedelta
+from typing import Optional
+
+# Primary configuration constants
+SEARCH_WINDOW_MONTHS = 6  # Default seasonal scan window
+SAMPLE_DAY_OF_MONTH = 15  # Representative mid-month date for seasonal queries
+
+
+def resolve_dates(date_arg: Optional[str], window: int) -> list[str]:
+    """
+    Resolve query dates based on CLI input.
+
+    Returns a single date if --date is provided; otherwise generates one date
+    per month across the window for seasonal scanning.
+
+    Args:
+        date_arg: Optional date string in YYYY-MM-DD format
+        window: Number of months to scan (only used if date_arg is None)
+
+    Returns:
+        List of date strings in YYYY-MM-DD format
+    """
+    if date_arg:
+        return [date_arg]
+
+    today = date.today()
+    dates = []
+
+    for i in range(1, window + 1):
+        # Generate dates starting from next month
+        target_date = today + relativedelta(months=i)
+        # Set to the sample day of month (default: 15th)
+        try:
+            target_date = target_date.replace(day=SAMPLE_DAY_OF_MONTH)
+        except ValueError:
+            # Handle months with fewer days (e.g., February with day=31)
+            # Use the last day of the month instead
+            target_date = target_date.replace(day=1) + relativedelta(months=1) - relativedelta(days=1)
+
+        dates.append(target_date.strftime('%Y-%m-%d'))
+
+    return dates
+
+
+def resolve_dates_daily(
+    start_date: Optional[str],
+    end_date: Optional[str],
+    window: int,
+) -> list[str]:
+    """
+    Generate a list of ALL dates in a range, day-by-day (Monday-Sunday).
+
+    This is used for comprehensive daily scans to catch flights that only operate
+    on specific days of the week (e.g., Saturday-only routes).
+
+    Args:
+        start_date: Optional start date in YYYY-MM-DD format
+        end_date: Optional end date in YYYY-MM-DD format
+        window: Number of months to scan if dates not specified
+
+    Returns:
+        List of date strings in YYYY-MM-DD format, one per day
+
+    Examples:
+        # Scan next 3 months daily
+        resolve_dates_daily(None, None, 3)
+
+        # Scan specific range
+        resolve_dates_daily("2026-04-01", "2026-06-30", 3)
+    """
+    today = date.today()
+
+    # Determine start date
+    if start_date:
+        start = date.fromisoformat(start_date)
+    else:
+        # Start from tomorrow (or first day of next month)
+        start = today + timedelta(days=1)
+
+    # Determine end date
+    if end_date:
+        end = date.fromisoformat(end_date)
+    else:
+        # End after window months
+        end = today + relativedelta(months=window)
+
+    # Generate all dates in range
+    dates = []
+    current = start
+    while current <= end:
+        dates.append(current.strftime('%Y-%m-%d'))
+        current += timedelta(days=1)
+
+    return dates
+
+
+def detect_new_connections(monthly_results: dict[str, list]) -> dict[str, str]:
+    """
+    Detect routes that appear for the first time in later months.
+
+    Compares route sets month-over-month and tags routes that weren't present
+    in any previous month.
+
+    Args:
+        monthly_results: Dict mapping month strings to lists of flight objects
+                        Each flight must have 'origin' and 'destination' attributes
+
+    Returns:
+        Dict mapping route keys (e.g., "FRA->JFK") to the first month they appeared
+    """
+    seen = set()
+    new_connections = {}
+
+    # Process months in chronological order
+    for month in sorted(monthly_results.keys()):
+        flights = monthly_results[month]
+        current = {f"{f['origin']}->{f['destination']}" for f in flights}
+
+        # Find routes in current month that weren't in any previous month
+        for route in current - seen:
+            if seen:  # Only tag as NEW if it's not the very first month
+                new_connections[route] = month
+
+        # Add all current routes to seen set
+        seen |= current
+
+    return new_connections