Add Langfuse observability and LLM response caching

[HazalCiplak]

https://github.com/eLifePathways/ScienceBeam2.0/issues/33

[de-code]

The first time I tried to start it I got this:

langfuse-db-1 | 2026-04-23 15:44:34.811 UTC [1] LOG: starting PostgreSQL 16.13 on x86_64-pc-linux-musl, compiled by gcc (Alpine 15.2.0) 15.2.0, 64-bit
langfuse-db-1 | 2026-04-23 15:44:34.811 UTC [1] LOG: listening on IPv4 address "0.0.0.0", port 5432
langfuse-db-1 | 2026-04-23 15:44:34.811 UTC [1] LOG: listening on IPv6 address "::", port 5432
langfuse-db-1 | 2026-04-23 15:44:34.822 UTC [1] LOG: listening on Unix socket "/var/run/postgresql/.s.PGSQL.5432"
langfuse-db-1 | 2026-04-23 15:44:34.834 UTC [57] LOG: database system was shut down at 2026-04-23 15:44:34 UTC
langfuse-db-1 | 2026-04-23 15:44:34.842 UTC [1] LOG: database system is ready to accept connections
langfuse-1 | Error: P1001
langfuse-1 |
langfuse-1 | Can't reach database server at langfuse-db:5432
langfuse-1 |
langfuse-1 | Please make sure your database server is running at langfuse-db:5432.
langfuse-1 |
langfuse-1 | Prisma schema loaded from packages/shared/prisma/schema.prisma
langfuse-1 | Datasource "db": PostgreSQL database "langfuse", schema "public" at "langfuse-db:5432"
langfuse-1 |
langfuse-1 | Error: P1001: Can't reach database server at langfuse-db:5432
langfuse-1 |
langfuse-1 | Please make sure your database server is running at langfuse-db:5432.
langfuse-1 | Applying database migrations failed. This is mostly caused by the database being unavailable.
langfuse-1 | Exiting...

[de-code]

Took me a while to figure out (with help from Claude Code) that I had to re-create the container. I didn't even check whether it was still around.

After that it was working fine and I can see the information I am looking for.

Memory consumption seem to be ~204 MB + 69 MB for the database:

CONTAINER ID   NAME                                                          CPU %     MEM USAGE / LIMIT     MEM %     NET I/O           BLOCK I/O         PIDS
d9ea08aa8962   sciencebeam-grobid-metadata-enricher-langfuse-langfuse-1      0.00%     204.8MiB / 62.25GiB   0.32%     1.08MB / 2.12MB   0B / 0B           36
032d3f6f13f5   sciencebeam-grobid-metadata-enricher-langfuse-langfuse-db-1   0.00%     69.07MiB / 62.25GiB   0.11%     1.04MB / 791kB    6.91MB / 93.4MB   15