refactor(sync): consolidate local SQLite under PowerSync

[orga-agent]

What

Finish plan #1783 by making the daemon's PowerSyncDatabase the single local SQLite ownership boundary.

• replace local SQLx SQLite access with LocalPowerSyncBackend over the daemon's shared PowerSync database
• remove the SQLx dependency, pool, metadata, prepare scripts, CI flags, and dev tooling
• upgrade to the finalized pn 0.0.7 candidate and rusqlite 0.39
• remove Application write signals, the manual mpsc upload worker, startup drain, upload guard, and duplicate retry loop
• rely on PowerSync's tested update notifications, initial CRUD scan, and retry actor
• retain periodic/startup/shutdown WAL checkpoints as disk maintenance
• update Nix with the libclang and CA inputs required by the new pn build/TLS stack

Net result: one daemon process, one PowerSync SQLite pool, one upload actor, and about 1,227 fewer lines in this branch's second commit.

Tests

• cargo test --workspace --all-features — 321 tests passed
• cargo check --workspace --all-targets --all-features — passed
• strict workspace/all-target/all-feature Clippy — passed
• cargo fmt --all --check — passed
• cargo deny check — passed
• cargo build --workspace — passed
• bash scripts/test-release.sh — passed
• nix build .#flicknote --no-link — passed
• actor contracts passed for live writes, CRUD queued before connect, and transient upload retry
• existing same-file schema/rollback/daemon/MCP contracts passed

Smoke

Using the real dev configuration:

• branch daemon started and reported protocol 2
• note 1817 was created and immediately read
• ps_crud=0 after create
• note 1817 was archived and ps_crud=0 again
• the original installed daemon was restored

Dependency

Depends on GuionAI/powersync-native#3. This branch pins its finalized candidate commit 133f7eabacfbd49b3240f8313807105eab0ef315, including the reviewed SQLite cleanup and binding fixes. After pn PR #3 merges and v0.0.7-guion.1 is published, this PR will switch from the commit pin to that tag before merge.

Plan: FlickNote #1783.