fix: restore ppc64le compatibility by updating SQLx

[Nameless-Monster-Nerd]

Fixes #686

Summary

• update SQLx from 0.6 to 0.8.6 and use its SQLite-only, TLS-free runtime features
• remove the unused jsonwebtoken dependency that also pulled in ring 0.16
• adapt the SQLite connection options call to SQLx 0.8
• pin the active signature 1.x dependency required by the existing ed25519/sodiumoxide stack

The active dependency graph no longer contains ring 0.16, which is the version that fails on ppc64le.

Validation

• cargo check --locked
• cargo test --locked --no-run
• rustfmt --edition 2021 --check src/database.rs
• cargo tree --locked -i ring@0.16.20 (no dependency found)

The repository-wide cargo fmt --check still reports formatting drift in pre-existing unrelated files, including the pinned submodule.

AI assistance was used to analyze the dependency graph and prepare this patch; I reviewed the changes and validation results.

[coderabbitai]

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