Allow PG database backend

[durch]

Summary

• Added PostgreSQL database support alongside existing SQLite through Cargo feature flags
• Implemented runtime query conversion from SQLite ? placeholders to PostgreSQL $1, $2, ... format
• Single codebase now supports both databases without query duplication

Implementation

• Created query wrapper functions (query(), query_as(), query_scalar()) that automatically convert placeholders
• Converted all sqlx macro calls to use the wrapper functions
• Added type conversions where needed (SQLite i64 → PostgreSQL i32)
• Feature-gated database-specific SQL syntax (RETURNING clauses, INSERT OR IGNORE vs ON CONFLICT)

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[dynco-nym]

CI is failing due to clippy, but I couldn't fix it locally because cargo build is failing

[dynco-nym]

great work BTW, looking forward to having this merged!