Fold COASTAL_BATTERY into HOSTILE_ARTILLERY, it's the same thing

Same pattern already established for AMMO_CACHE -> SUPPLY_CACHE: a
'Coastal Battery' is just a heavy fixed-emplacement HostileArtillery
under a different name in the game's own text, not a meaningfully
different unit type worth its own enum member. Removed the
TargetType.COASTAL_BATTERY member, added 'CoastalBattery' to
ocr.py's _TYPE_WORD_ALIASES (so 'Coastal Battery#2:' still parses,
now as a HostileArtillery), and a models.py migration entry so any
save file written before this change still loads correctly.

Verified the migration directly (COASTAL_BATTERY -> HOSTILE_ARTILLERY,
existing AMMO_CACHE -> SUPPLY_CACHE unaffected) and updated the one
test that asserted the old type.

tests/test_ocr.py

@@ -94,14 +94,17 @@ def test_enemy_destroyed_report():
 
 
 def test_listening_post_and_coastal_battery():
+    """'Coastal Battery' is an alias for HostileArtillery (see
+    _TYPE_WORD_ALIASES), not its own TargetType, it's just the
+    fixed-emplacement flavor of the same thing."""
     text = """Listening Post#1 at K6 7:8 audio reports on:
 Coastal Battery#2:
    Distance 6.28km South-East from Listening Post#1
 """
     info = ocr.parse_text(text)
     assert info.reference_points["ListeningPost#1"][2] == Coord("K", 6, 7, 8)
-    assert (TargetType.COASTAL_BATTERY, "2") in info.targets
-    _, clues, *_ = info.targets[(TargetType.COASTAL_BATTERY, "2")]
+    assert (TargetType.HOSTILE_ARTILLERY, "2") in info.targets
+    _, clues, *_ = info.targets[(TargetType.HOSTILE_ARTILLERY, "2")]
     assert clues == [ocr.Clue(reference="ListeningPost#1", bearing_deg=135.0, distance_km=6.28)]