Fix database schema mismatches between schema.sql and Rust implementation

[Copilot]

The cache layer's SQL schema definition (schema.sql) and Rust implementation (mod.rs) were out of sync, causing runtime inconsistencies. The schema file defined a proper relational structure with integer primary keys and foreign key constraints, while the Rust code used a denormalized TEXT-based approach.

Database Schema Fixes

• trace_files table: Changed from file_path TEXT PRIMARY KEY to id INTEGER PRIMARY KEY with file_path TEXT NOT NULL UNIQUE
• edit_records table: Changed from trace_file_path TEXT to trace_file_id INTEGER NOT NULL with FOREIGN KEY (trace_file_id) REFERENCES trace_files(id)
• Type corrections: file_mtime_ns and file_size_bytes now stored as INTEGER instead of TEXT

// Before: denormalized, text-based
CREATE TABLE trace_files (
    file_path TEXT PRIMARY KEY,
    file_mtime_ns TEXT NOT NULL,  // ❌ string conversion
    ...
)

// After: normalized, proper types
CREATE TABLE trace_files (
    id INTEGER PRIMARY KEY,
    file_path TEXT NOT NULL UNIQUE,
    file_mtime_ns INTEGER NOT NULL,  // ✅ native integer
    ...
)

Configuration Updates

• CSP header: Added connect-src 'self' tauri: and font-src 'self' to support Tauri IPC and font loading
• Build documentation: Clarified PNG placeholder is temporary, referencing proper icon setup in src-tauri/icons/README.md

Documentation

• README: Added note explaining PyPI distribution provides a Rust binary, not a Python library (no API bindings)

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