Limit MySQL discovery to the configured database

[just-some-random-pal]

mysql.database now scopes MySQL discovery the way MySQL Setup already says it does. Before, the setting only chose the connection's default database, and schema discovery scanned every non-system database on the server. Closes #124.

What changes:

• run_analysis passes {"database": ...} to every MySQL analyzer, so the setting is available where the queries are built.
• MySQLSchemaAnalyzer returns the configured database from _get_database_list without running SHOW DATABASES, and every information_schema query compares table_schema to that database instead of excluding the system databases.
• MySQLFeatureAnalyzer applies the same filter to its schema-based checks (full-text indexes, spatial columns, foreign keys, partitions, InnoDB compression), so a feature in another database is no longer reported for the configured one.
• An empty database keeps the current server-wide behavior, which the README and config template already describe as "discover all databases".
• Configuration, performance, replication, and security checks stay server-wide, and the doc now says so, because those describe the instance rather than a schema.

The filter embeds the configured name as a hex literal (table_schema = 0x6170...) rather than a quoted string. The fragment is spliced into many queries that execute_query runs without parameters, and a hex literal keeps the name out of SQL syntax regardless of the connection's escaping mode. Left as a plain literal it has the same coercibility as the quoted system database names the file already compares against information_schema, so it takes the column's character set on every supported server rather than depending on a utf8mb3/utf8mb4 tie-break. A unit test checks that app' OR 1=1 reaches the server only as hex.

Verification:

• Unit tests: five new cases in test_mysql_discovery.py, test_mysql_schema_analyzer.py, and test_mysql_feature_analyzer.py. The full suite, black, flake8, mypy, and bandit pass with the CI settings.
• End to end against a MySQL 8.0 container with an app database and an other_app database that has a full-text index: with database: app the report lists only app in database_catalog and every schema list, and full_text_indexing is false. With database: "" both databases appear and full_text_indexing is true.
• The hex literal matches a database name containing a quote and one containing non-ASCII characters, both against information_schema.schemata and against a utf8mb3_general_ci column, with the connection in utf8mb4 and in utf8mb3.