refactor(databases): state the databases.csv contract once

The schema lived three times in this repo alone: the seed and the backup each spelled out the column list and the semicolon, and _validate_database_value existed twice under one name with different strictness - the seed checked the character set, the backup only checked for empty. A hand-edited file therefore bypassed the only real validation on its way into a dump command.

baudolo.databases now holds the columns, the delimiter, the cluster marker, the validator and a read_rows() for consumers that do not want pandas. Values come back verbatim: a password may begin or end with a space, so stripping belongs to the caller that compares, never to the reader. DatabasesCsvError subclasses ValueError, so callers that predate the module keep catching what they caught.

The backup's call sites switch over in the next commit, which rewrites them anyway.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/baudolo/backup/dumps.py

@@ -7,6 +7,8 @@ import sys
 import pandas
 from pandas.errors import EmptyDataError
 
+from baudolo.databases import COLUMNS, DELIMITER
+
 from .db import backup_database
 from .docker import has_tool, image_id
 
@@ -81,7 +83,7 @@ def _empty_databases_df() -> pandas.DataFrame:
     This allows the backup to continue without DB dumps when the CSV is missing
     or empty (pandas EmptyDataError).
     """
-    return pandas.DataFrame(columns=["instance", "database", "username", "password"])
+    return pandas.DataFrame(columns=list(COLUMNS))
 
 
 def load_databases_df(csv_path: str) -> pandas.DataFrame:
@@ -93,7 +95,9 @@ def load_databases_df(csv_path: str) -> pandas.DataFrame:
     - Valid CSV        -> return dataframe
     """
     try:
-        return pandas.read_csv(csv_path, sep=";", keep_default_na=False, dtype=str)
+        return pandas.read_csv(
+            csv_path, sep=DELIMITER, keep_default_na=False, dtype=str
+        )
     except FileNotFoundError:
         print(
             f"WARNING: databases.csv not found: {csv_path}. Continuing without database dumps.",