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.