feat: add DuckDB telemetry backend for self-hosted mode (#203)

[jstojiljkovic]

Adds DuckDB as an opt-in telemetry store for self-hosted deployments (-tags duckdb, CGO required). The main DB stays SQLite (relational/config); only the telemetry DB becomes DuckDB (columnar), written via the Appender API.
Closes #203.

Three mutually-exclusive backends, selected at build time:

Build tag	Backend
pgch	Postgres + ClickHouse
!pgch && !duckdb	dual SQLite
duckdb && !pgch	SQLite main + DuckDB telemetry

Benchmarks (CCX13 · €16.49/mo · vs SQLite)

Read: largest fill level the dashboard stays usable (≤ 5 s, no timeout):

Signal	SQLite	DuckDB
Metrics	1M	10M
Spans	100k	10M
Logs	100k	10M

DuckDB holds 10–100× more queryable rows and flips SQLite's logs cliff.

Write: max sustainable items/sec:

Signal	SQLite	DuckDB	Speedup
Metrics	61,712	85,752	1.4×
Spans	30,508	61,587	2.0×
Logs	4,877	30,480	6.3×

Disk: ~6× smaller than SQLite (auto FSST/Dictionary compression).

[dusanstanojeviccs]

We have too many separate backends that our folder structure makes no sense anymore.

Instead of doing
repositories/something_duckdb.go
repositories/something_sqlite.go

We should do 3 folders:

repositories/telemetry/duckdb/something.go
repositories/telemetry/sqlite/something.go
repositories/telemetry/clickhouse/something.go

This will allow us to (based on tags) switch between them

We should also update the tags we use when building the projects, right now they are kinda confusing, we should probably do something like
telemetry_ch
telemetry_duckdb
telemetry_sqlite

oltp_sqlite
oltp_pg

This would make tags clear and allow us to implement more backends like victoria metrics/logs/traces easily

[jstojiljkovic]

We have too many separate backends that our folder structure makes no sense anymore.

Instead of doing repositories/something_duckdb.go repositories/something_sqlite.go

We should do 3 folders:

repositories/telemetry/duckdb/something.go repositories/telemetry/sqlite/something.go repositories/telemetry/clickhouse/something.go

This will allow us to (based on tags) switch between them

We should also update the tags we use when building the projects, right now they are kinda confusing, we should probably do something like telemetry_ch telemetry_duckdb telemetry_sqlite

oltp_sqlite oltp_pg

This would make tags clear and allow us to implement more backends like victoria metrics/logs/traces easily

@dusanstanojeviccs Agreed, separated everything with clean tags, so in the future we can easily add new backends