Add pg_dump/pg_restore backup and restore scripts

For moving this project to a new server: git tracks code, not the
Postgres data directory, and raw volume/data-dir copying is fragile
across Postgres versions or architectures (this repo already hit a
collation-version mismatch once from just changing the Postgres
image). pg_dump/pg_restore is the portable way to move the actual
data instead.

scripts/backup-db.sh dumps the running postgres container's database
in pg_dump's custom format into ./backups/ (gitignored — dumps
contain real user data and don't belong in git history). It reads
POSTGRES_USER/POSTGRES_DB from the container's own environment rather
than re-parsing .env, so it can't drift out of sync with the actual
running config.

scripts/restore-db.sh restores a dump into the running container
(--clean --if-exists --single-transaction, so a failure rolls back
atomically instead of leaving a half-restored database), with a
confirmation prompt since it's destructive, skippable via -y.

Added .gitattributes forcing LF on *.sh — Windows git would otherwise
check these out as CRLF, breaking the bash shebang on the Linux
server they're meant to run on.

Verified end to end: took a real backup of the live database, restored
it back over itself, and confirmed exact row-count matches across every
table plus that pgvector similarity search still works on the restored
embeddings (not just that the bytes round-tripped).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitattributes

@@ -0,0 +1,2 @@
+* text=auto
+*.sh text eol=lf