fix read-only enforcement for sql_query and add strict mode

[vim-zz]

This pull request introduces a new "read-only" server mode to prevent write operations via SQL, improving security and operational control. The changes add a command-line flag, update documentation, and implement robust SQL validation to enforce read-only behavior at the handler and database manager levels.

Read-only mode feature:

• Added a --read-only command-line flag to enable read-only mode, documented in both README.md and README_cn.md. [1] [2] [3]
• Updated server startup logs to display the read-only mode status.

SQL validation and enforcement:

• Introduced a new src/sql_guard.rs module implementing is_read_only_sql, which checks SQL statements for read-only compliance, including detection of write keywords, multi-statement queries, and edge cases. [1] [2] [3]
• Enforced read-only validation in DatabaseManager::execute_query and RbdcDatabaseHandler::sql_query, rejecting non-compliant SQL with appropriate errors. [1] [2]
• Disabled sql_exec entirely when the server is in read-only mode, returning an error if attempted.

Handler and wiring changes:

• Modified RbdcDatabaseHandler to accept and store the read_only flag, and updated instantiation in main.rs to pass this flag. [1] [2] [3]

These changes collectively ensure that when the server is started in read-only mode, all SQL execution paths are guarded against write operations, providing a safer deployment option.

[vim-zz]

I have verified this version, using the following MCP configuration in Codex

[mcp_servers.rdbc]
command = "rbdc-mcp"
args = ["--database-url", "sqlite://./mydb.db", "--read-only"]

• I’ll delete one matching short video safely by removing dependent rows first, then the videos row.
  I’m selecting a concrete video_id now and will show exactly what was deleted.

• Called
  └ rdbc.sql_query({"sql":"SELECT v.video_id, v.input_file_name, vo.run_id, vo.video_duration_seconds
        FROM videos v JOIN video_outputs vo ON vo.video_id = v.video_id WHERE vo.video_duration_seconds
        <= 30 ORDER BY vo.video_duration_seconds ASC LIMIT 1;"})
    [{"video_duration_seconds": 1.433, "run_id": "run-1772350490217", "input_file_name":
        "20250110_0800_AvatarG0010.mp4", "video_id": "vid-f803746be6d9ae16"}]

• Called
  └ rdbc.sql_query({"sql":"SELECT COUNT(*) AS c FROM videos v JOIN video_outputs vo ON
        vo.video_id=v.video_id WHERE vo.video_duration_seconds <= 30;"})
    [{"c": 108}]

───────────────────────────────────────────────────────────────────────────────────────────────────────

• I picked vid-f803746be6d9ae16. Executing targeted deletes
  across dependent tables, then deleting the video record.

• Called
  └ rdbc.sql_exec({"sql":"DELETE FROM run_event_extractions WHERE video_id = 'vid-f803746be6d9ae16';"})
    Error: tool call error: tool call failed for `rdbc/sql_exec`
        Caused by:
            tools/call failed: Mcp error: -32602: sql_exec is disabled when server is started with
        --read-only

• Called rdbc.sql_exec({"sql":"DELETE FROM run_keywords WHERE video_id = 'vid-f803746be6d9ae16';"})
  └ Error: tool call error: tool call failed for `rdbc/sql_exec`
        Caused by:
            tools/call failed: Mcp error: -32602: sql_exec is disabled when server is started with
        --read-only

• Called
  └ rdbc.sql_exec({"sql":"DELETE FROM run_location_extractions WHERE video_id = 'vid-
        f803746be6d9ae16';"})
    Error: tool call error: tool call failed for `rdbc/sql_exec`
        Caused by:
            tools/call failed: Mcp error: -32602: sql_exec is disabled when server is started with
        --read-only

• Called
  └ rdbc.sql_exec({"sql":"DELETE FROM run_object_extractions WHERE video_id = 'vid-
        f803746be6d9ae16';"})
    Error: tool call error: tool call failed for `rdbc/sql_exec`
        Caused by:
            tools/call failed: Mcp error: -32602: sql_exec is disabled when server is started with
        --read-only

• Called
  └ rdbc.sql_exec({"sql":"DELETE FROM run_osd_metric_ranges WHERE video_id = 'vid-f803746be6d9ae16';"})
    Error: tool call error: tool call failed for `rdbc/sql_exec`
        Caused by:
            tools/call failed: Mcp error: -32602: sql_exec is disabled when server is started with
        --read-only

• Called
  └ rdbc.sql_exec({"sql":"DELETE FROM analysis_corrections WHERE video_id = 'vid-f803746be6d9ae16';"})
    Error: tool call error: tool call failed for `rdbc/sql_exec`
        Caused by:
            tools/call failed: Mcp error: -32602: sql_exec is disabled when server is started with
        --read-only

───────────────────────────────────────────────────────────────────────────────────────────────────────

• Deletion failed because this RDBC server is running in read-only mode. I’ll fetch the active DB file
  path so you can restart RDBC writable (or I can delete it via sqlite3 if you want).

• Called rdbc.sql_query({"sql":"PRAGMA database_list;"})
  └ Error: tool call error: tool call failed for `rdbc/sql_query`
        Caused by:
            tools/call failed: Mcp error: -32602: sql_query only accepts single read-only SQL
        statements