fix: make get_db_path() idempotent to prevent recursive _dev suffix

Root Cause:
The get_db_path() function was being called multiple times in the
initialization chain, causing recursive suffix addition:
  data/jobs.db -> data/jobs_dev.db -> data/jobs_dev_dev.db

This resulted in tables being created in data/jobs_dev_dev.db while
the application tried to access data/jobs_dev.db (empty database).

Fix:
Added idempotency check in get_db_path() to detect and skip
transformation if path already contains "_dev.db" suffix.

Evidence from Diagnostics:
- alpha.20 logs showed: Input='data/jobs_dev.db', Resolved='data/jobs_dev_dev.db'
- Tables were created in jobs_dev_dev.db but accessed from jobs_dev.db
- This caused "no such table: jobs" errors despite successful initialization

All 28 integration tests pass with this fix.

Fixes #6

tools/deployment_config.py

@@ -69,8 +69,16 @@ def get_db_path(base_db_path: str) -> str:
     Example:
         PROD: "data/trading.db" -> "data/trading.db"
         DEV:  "data/trading.db" -> "data/trading_dev.db"
+
+    Note:
+        This function is idempotent - calling it multiple times on the same
+        path will not add multiple _dev suffixes.
     """
     if is_dev_mode():
+        # Check if already has _dev suffix (idempotent)
+        if "_dev.db" in base_db_path:
+            return base_db_path
+
         # Insert _dev before .db extension
         if base_db_path.endswith(".db"):
             return base_db_path[:-3] + "_dev.db"