docs: consolidate database videos into tutorials and harden quick examples

[bartlomieju]

Three small quality passes on the examples catalog that were parked
during the recent expansion, all from the consolidation section of the
content backlog.

The Mongoose and Prisma video pages duplicated their tutorials: same
topic, two catalog entries, and the relabeled titles ("Prisma" next to
"Prisma (video)") made the redundancy obvious. The videos now embed at
the top of their tutorials, the standalone video pages are deleted, and
the retired URLs redirect to the tutorials, the same approach as the
earlier compress_decompress fold. The catalog drops from 316 to 314
items. Redis keeps both of its pages since the quick start (connect and
send commands) and the tutorial (build a caching layer) cover different
ground; they now cross-link each other instead.

The Postgres quick example previously hardcoded credentials and let
connection failures escape as a raw stack trace. It now reads the
standard PG* environment variables with sensible localhost fallbacks and
handles query failure by reporting the error code; the failure path was
verified against a closed port (prints "Query failed: ECONNREFUSED") and
the pool is closed in a finally block either way.

The project initialization tutorial showed bare commands with no
expected output and had drifted from what deno init actually generates
(it described the old "add function" template; the current template is a
small HTTP server with two tests, and the dev task includes
--allow-net). The tutorial now shows the real captured output of deno
init, deno task dev, and deno test, so readers can verify each step as
they follow along.

Embedded video on the Mongoose tutorial: