Implement story 2.1: aircraft data model and fetcher

Add HttpFetcher and FileFixtureFetcher with shared _parse_aircraft helper,
DUMP1090_URL constant, realistic fixture data, and full test coverage for
all acceptance criteria (AC1–AC5).

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

src/planemapper/constants.py

@@ -1,5 +1,7 @@
 from pathlib import Path
 
+DUMP1090_URL = "http://localhost:8080/data/aircraft.json"
+
 DISPLAY_WIDTH = 800
 DISPLAY_HEIGHT = 480
 

src/planemapper/fetcher.py

@@ -1,7 +1,46 @@
+import json
+from pathlib import Path
 from typing import Protocol
 
+import requests
+
+from planemapper.constants import DUMP1090_URL, FETCH_TIMEOUT_S
 from planemapper.models import Aircraft
 
 
 class FetcherInterface(Protocol):
     def fetch(self) -> list[Aircraft]: ...
+
+
+def _parse_aircraft(entry: dict) -> Aircraft:
+    raw_alt = entry.get("altitude", 0)
+    altitude_ft = int(raw_alt) if isinstance(raw_alt, int) else 0
+    return Aircraft(
+        icao=entry["hex"],
+        lat=float(entry["lat"]),
+        lon=float(entry["lon"]),
+        heading=float(entry.get("track", 0.0)),
+        altitude_ft=altitude_ft,
+        callsign=entry.get("flight", "").strip(),
+        category=entry.get("category", ""),
+        is_mlat=bool(entry.get("mlat")),
+        is_stale=False,
+    )
+
+
+class HttpFetcher:
+    def fetch(self) -> list[Aircraft]:
+        resp = requests.get(DUMP1090_URL, timeout=FETCH_TIMEOUT_S)
+        resp.raise_for_status()
+        data = resp.json()
+        return [_parse_aircraft(e) for e in data.get("aircraft", []) if "lat" in e and "lon" in e]
+
+
+class FileFixtureFetcher:
+    def __init__(self, path: Path) -> None:
+        self._path = path
+
+    def fetch(self) -> list[Aircraft]:
+        with self._path.open() as f:
+            data = json.load(f)
+        return [_parse_aircraft(e) for e in data.get("aircraft", []) if "lat" in e and "lon" in e]