Fix backend lint errors in PostgresConnector and statistics endpoint

[cheezyrock]

Motivation

• Resolve linting errors: C0321 (multiple-statements), E0203 (access-member-before-definition), and E1120 (missing parameter) reported for backend modules.

Description

• Initialize PostgresConnector.connection to None in __init__ and simplify the connect() guard to avoid accessing the member before definition and ensure a clear connection check.
• Expand single-line conditionals and inline statements in Backend/postgres_connector.py (rollback handling, polynomial monomial construction, and string concatenation branches) into multi-line blocks to satisfy C0321 style rules.
• Update the /get_statistics route in Backend/server.py to call connector.build_where_text(filters) and pass both where_sql and params into connector.get_statistics() to supply the required arguments.

Testing

• Successfully compiled the modified modules with python -m py_compile Backend/postgres_connector.py Backend/server.py.
• Attempted to run pylint Backend/postgres_connector.py Backend/server.py but pylint is not available in the execution environment, so lint checks could not be re-run here.

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Codex Task