Postgresql support

[finjaadam]

• Add PostgreSQL support to the content-dump tool, mirroring the existing MySQL/MariaDB path: full schema reconstruction (columns/types, NOT NULL, defaults, identity/generated columns, PK/UNIQUE/CHECK/FK constraints, indexes, sequence state) from pg_catalog, with data streamed via COPY ... FROM stdin — no pg_dump/psql binary shelled out to, keeping synco self-contained.
  • Wire driver detection (pdo_pgsql/pgsql) through Flow and Laravel credential extraction, the shared DatabaseDump dispatcher, and the receive download switch (dto.TYPE_POSTGRESDUMP, previously an unimplemented stub).
  • Fix two bugs found in review along the way: an ignored EncryptToFile error that could panic on a nil writer, and unescaped single quotes in the setval sequence-name literal.
  • Document known limitations (custom types/extensions, non-public schemas, materialized views, partitioned tables, triggers/functions, comments, GRANTs — not dumped) in docs/README.md/docs/architecture.md.

Test plan

• Unit tests for DDL/constraint/index/sequence/COPY generation (pkg/util/postgres/go_pgdump, sqlmock-based) and driver detection (flowServe, laravelServe)
• New e2e test (test_e2e/postgresframework_test.go) using gnomock's Postgres preset, mirroring the existing MariaDB e2e test
• Manually verified end-to-end against a real local Neos/Flow-on-Postgres project: serve/receive round-trip, dump replayed into a scratch Postgres via psql -f, row counts matched, and a post-restore insert confirmed no identity/sequence collision