Fix Docker runtime environment, SQLite pathing, and documentation

[bmcdonald3]

Summary of changes:

• Docker image migration: Transitioned the base image from debian:bookworm-slim to gcr.io/distroless/static-debian12:nonroot. Introduced a builder stage to initialize a /data directory and assign ownership to the nonroot user (UID/GID 65532) for persistent SQLite storage.
• Default configuration: Updated the DatabaseURL default value in cmd/server/main.go to target /data/fru-tracker.db, ensuring the application writes to the initialized, writable directory by default.
• Database documentation: Added docs/db.md to document the current Ent ORM startup auto-migration strategy, the supported SQLite driver, and the future transition plan toward versioned migration files.
• README enhancements: Added a specific warning under the verification steps regarding the asynchronous operation of the reconciler to set expectations for state delays. Reformatted the curl POST command into a single line.