Remove SQLite support: Piri now requires Postgres (Curio harmonydb) #35

Context

With the adoption of Curio's PDP core (#22), Piri's persistence runs on Curio's harmonydb/harmonytask engine, which is Postgres-only (Yugabyte pgx dialect, triggers, harmony task scheduling). SQLite can no longer back a Piri node, but the code and docs still present it as a supported option.

Work

Docs — remove/rewrite SQLite as a database option; state the Postgres requirement:

• docs/content/concepts/database.md
• docs/content/concepts/index.md
• docs/content/configuration/repo/database.md
• docs/content/operator-guide/storage.md

Code — remove SQLite support:

• pkg/database/sqlitedb (and its tests)
• SQLite branches in the config surface (pkg/config, pkg/config/app) and any fx wiring that selects a SQLite backend
• Setup/registration paths that reference SQLite (cmd/cli/setup/register.go)

Note on the jobqueues

lib/jobqueue and the aggregation pipeline queues (pkg/pdp/aggregation/{manager,commp,aggregator}/jobqueue.go) are also SQLite-backed today, but they are not in scope here: #30 replaces them with harmonytask tasks (PDPCommP, PDPAggregate, PDPAddRoots, PDPRemoveSweep) with all state in harmonydb, retiring the separate aggregator_db. This issue covers whatever SQLite surface remains after #30 lands — primarily the operator-facing database configuration, pkg/database/sqlitedb, and the docs.