feat(cli): connectivity checks for DB and embedding in interactive init

[aptend]

What type of PR is this?

• feat (new feature)
• chore (maintenance, tooling)

Which issue(s) this PR fixes

N/A — follow-up to #41

What this PR does / why we need it

Adds connectivity validation to memoria init -i after user confirms settings, before writing config files.

Connectivity checks

Between the confirm step and writing config:

• Database: TCP connect to host:port (3s timeout) — catches wrong host/port/DB not running
• Embedding: POST /embeddings with {"model":"...","input":"test"} — verifies URL reachable, API key valid, model exists

On failure, shows the error and prompts Continue anyway? so users can still write config if they plan to start services later.

Other changes

• Fix clippy collapsible_if warning
• Add scripts/migrate_embedding_dim.sh to .gitignore (local-only utility)

Changes

File	Change
memoria/crates/memoria-cli/src/main.rs	check_db, check_embedding, check_embedding_request functions; fix read_password_line tty fallback; clippy fix
.gitignore	Ignore local migration script