Fix SQLite startup crash dropping workspaces.package_manager on populated databases

[jbouder]

Fixes #549

Problem

Since #522, every SQLite-backed nebi (desktop app and nebi serve) crashes on startup when migrating a database created before the package_manager removal that contains workspaces referenced by child rows (jobs, publications, …):

Error: failed to run migrations: failed to drop workspaces.package_manager column: constraint failed: FOREIGN KEY constraint failed (787)

The glebarez/sqlite driver emulates DropColumn by rebuilding the table (create workspaces__temp → copy rows → DROP TABLE workspaces → rename), and — unlike its DropTable/AlterColumn paths — does not suspend foreign_keys first. With nebi's DSN-level _pragma=foreign_keys(ON), the DROP TABLE on the parent violates the foreign keys held by referencing rows and aborts startup. The rebuild runs inside a single transaction, so the failure rolls back cleanly — the database is left intact but cannot boot, and the crash recurs on every startup.

Postgres is unaffected (DropColumn is a native ALTER TABLE ... DROP COLUMN there).

Fix

• Run the package_manager drop pinned to a single connection (db.Connection) with PRAGMA foreign_keys = OFF/ON around it. Pinning is required because the DSN pragma re-enables enforcement on every new pooled connection, and the pragma must be flipped outside the rebuild's transaction, where SQLite ignores it.
• Keep the plain native drop on Postgres.

An earlier revision of this PR also dropped a "stranded" workspaces__temp table at startup, on the theory that failed attempts leave the rebuild's temp table behind. That premise was wrong — the rebuild is transactional and rolls the temp table back on failure (verified empirically and against a real post-crash database) — so that cleanup has been removed.

Testing

• New TestMigrateDropsLegacyPackageManagerColumnWithReferencingRows reproduces the crash scenario: legacy schema with the NOT NULL column and a jobs row holding an FK to a workspace. It verifies the migration succeeds, the column is gone, and all rows survive.
• Verified end-to-end against a real pre-Remove uv and the package-manager abstraction; pixi is the only package manager #522 nebi.db (legacy column + referencing job rows): nebi serve previously crashed, now boots cleanly with data intact.
• go test ./internal/db/, go build ./internal/..., go vet, gofmt all clean.

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