Remove ASTER, PUMP, ZEC from dashboard and stop tracking their history

- Remove ASTER, PUMP, ZEC from WATCHED_COINS in main_app.py
- Remove ASTER, PUMP, ZEC from coin_id_map.json (stops market cap collection)
- Remove ASTER, PUMP, ZEC from market_cap_data.json summary
- Remove ASTER, PUMP, ZEC manual overrides from coin_id_map.py
- Remove ASTER, PUMP, ZEC from resampling_status.json (gitignored)
- Add WIKI/symbol_management.md documentation for adding/removing symbols

Existing data in market_data.db is preserved; only new data collection stops.

_data/coin_id_map.json

@@ -16,7 +16,6 @@
     "AR": "arweave",
     "ARB": "osmosis-allarb",
     "ARK": "ark-3",
-    "ASTER": "astar",
     "ATOM": "lost-bitcoin-layer",
     "AVAX": "binance-peg-avalanche",
     "AVNT": "avantis",
@@ -139,7 +138,6 @@
     "POPCAT": "popcat",
     "PROMPT": "wayfinder",
     "PROVE": "succinct",
-    "PUMP": "pump-fun",
     "PURR": "purr-2",
     "PYTH": "pyth-network",
     "RDNT": "radiant-capital",
@@ -198,7 +196,6 @@
     "XRP": "ripple",
     "YGG": "yield-guild-games",
     "YZY": "yzy",
-    "ZEC": "zcash",
     "ZEN": "zenith-3",
     "ZEREBRO": "zerebro",
     "ZETA": "zeta",