fix(economy): handle db_error in commands, dedupe cooldowns, single leaderboard scan

Three robustness/perf fixes from the review:

1. db_error UX: economy core returns {"ok": False, "reason": "db_error"} on a
   PocketBase outage, but no handler expected it - deferred commands hung on
   "thinking..." and others showed a misleading "you're broke". Added a shared
   reply_db_error helper (commands/_replies.py) + S.ERR["db_error"], and wired a
   db_error branch into every handler that can receive it (daily/work/beg/crime/
   rob/give/buy/roulette/slots/blackjack/heist/fish/prestige/vanity/consumables/
   request-funding). Also fixed a latent KeyError in vs-bot RPS that read
   res["balance"] without checking res["ok"].

2. Deduped the item->cooldown mapping that was copied in do_daily/do_work/do_beg,
   do_fish_start, _maybe_remind and _restore_reminders. Single source of truth:
   store.ITEM_COOLDOWNS + effective_cooldown(cmd, items).

3. /leaderboard did six full-collection scans (one per tab). Added
   get_all_leaderboards() which scans once and builds all six views in memory.

Tests: effective_cooldown cases, and get_all_leaderboards matches the individual
queries + scans the collection exactly once.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013VbAVsrZuYesea99mPMmPT

commands/economy_fish_commands.py

@@ -10,6 +10,8 @@ from discord import app_commands
 from core import economy
 import strings as S
 
+from ._replies import reply_db_error
+
 
 def register_economy_fish_commands(
     tree: app_commands.CommandTree,
@@ -218,6 +220,9 @@ def register_economy_fish_commands(
 
         res = await economy.do_fish_start(interaction.user.id)
         if not res["ok"]:
+            if res["reason"] == "db_error":
+                await reply_db_error(interaction)
+                return
             if res["reason"] == "banned":
                 await interaction.response.send_message(S.MSG_BANNED, ephemeral=True)
             elif res["reason"] == "cooldown":