feat(server): migrate database layer from SQLite to PostgreSQL

[nasarajoe]

Summary

Sets up PostgreSQL as the persistent store for indexed blockchain data in server/.

Prisma was already installed with a complete 8-model schema, but the datasource was
bound to SQLite/libSQL. The work here is therefore a datasource conversion rather
than a from-scratch ORM install — the existing schema is preserved unchanged, and only
the database provider, driver adapter, and migration history are swapped to PostgreSQL.

Changes

File	Change
prisma/schema.prisma	Datasource provider sqlite → postgresql
src/database/database.module.ts	PrismaLibSql → PrismaPg adapter; now throws if DATABASE_URL is unset instead of silently falling back to a local dev.db file
prisma/migrations/20260819164517_init/	New PostgreSQL initial migration
prisma/migrations/migration_lock.toml	Provider → postgresql
src/config/env.validation.ts	DATABASE_URL is now required and must be a postgres(ql):// URI
src/config/configuration.ts	Removed the SQLite default from db.url
.env.example	Documents the PostgreSQL connection string format
package.json	Removed @libsql/client + @prisma/adapter-libsql; added @prisma/adapter-pg, pg, @types/pg
docker-compose.yml	New — PostgreSQL 16 with healthcheck and named volume

New scripts

npm run db:migrate / db:migrate:deploy / db:migrate:status / db:migrate:reset / db:studio

Verification

Run against a live PostgreSQL 16.14 instance:

• DB connects successfully — SELECT version() returned PostgreSQL 16.14; a
  Campaign create/read/delete round-tripped through the real PrismaPg adapter,
  confirming column defaults (status='Active', totalFunded=0) and BigInt handling.
• Migration runs without errors — prisma migrate status reports "Database schema
  is up to date!"; all 8 tables plus _prisma_migrations are created.
• Build passes — nest build exits 0.
• Tests pass — 58/58 across 4 suites.
• No libsql / sqlite / dev.db references remain outside generated client code.

Notes for reviewers

1. The migration history was reset — breaking for existing environments. The previous
SQLite migration used DATETIME and inline PRIMARY KEY syntax invalid in PostgreSQL, so
it could not be carried forward. Existing environments need npm run db:migrate:reset.

2. Default port is 55432, not 5432 — avoids colliding with a system PostgreSQL.

3. Compose credentials are local-development only; production supplies DATABASE_URL
via environment configuration.

Closes #24