feat(datasets): periods (start_date/end_date), dg_id column, date filters; rename observation dates

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

README.md

@@ -72,10 +72,10 @@ the KATO column and the year columns:
    `parse_error`).
 3. The client polls `GET /datasets/{id}` until `awaiting_mapping`, then submits
    `POST /datasets/{id}/mapping` with the chosen `kato_column` and a
-   `year_columns` map (each `{column, date}`). Validated against the detected
-   columns; `status` → `extracting`.
+   `year_columns` map (each `{column, start_date}`, optionally `end_date` for
+   a period). Validated against the detected columns; `status` → `extracting`.
 4. A second worker job **unpivots** the attribute table into long-format
-   `dataset_observations` — one row per `(kato_code, date)` with a numeric
+   `dataset_observations` — one row per `(kato_code, start_date)` with a numeric
    `value` (or `value_text` for non-numeric cells); `status` → `ready`. Read
    them via `GET /datasets/{id}/observations` (paginated, optional
    `?kato_code=`).
@@ -84,12 +84,25 @@ the KATO column and the year columns:
 curl -X POST https://dssgis.dwh.kz/datasets/<id>/mapping -H 'Content-Type: application/json' -d '{
   "kato_column": "като",
   "year_columns": [
-    {"column": "F_2023", "date": "2023-01-01"},
-    {"column": "D_2025", "date": "2025-01-01"}
+    {"column": "F_2023", "start_date": "2023-01-01"},
+    {"column": "D_2025", "start_date": "2025-01-01"}
   ]
 }'
 ```
 
+### Dataset periods
+
+A dataset carries an optional period — `start_date` and `end_date`
+(`YYYY-MM-DD`, both nullable, `end_date` inclusive and never before
+`start_date`) — plus an optional `dg_id` linking it to a *dataset group*: the
+"одноимённые" datasets that describe the same thing for different periods. All
+three are accepted on `POST /datasets` (form fields), `PUT /datasets/{id}` and
+`PATCH /datasets/{id}` (an explicit `null`, or omission on PUT, clears them).
+
+`GET /datasets` filters on them with `?dg_id=`, and with the inclusive bounds
+`?start_date_from=`, `?start_date_to=`, `?end_date_from=` and `?end_date_to=`
+(each `YYYY-MM-DD`).
+
 ## Getting started
 
 ```sh
@@ -117,13 +130,13 @@ server runs it is served at `/openapi.yaml`, with an interactive **Redoc** UI at
 | GET    | `/categories/{id}`         | get                                  |
 | PUT    | `/categories/{id}`         | update                               |
 | DELETE | `/categories/{id}`         | delete                               |
-| GET    | `/datasets`                | paginated list of summaries (`?page=`, `?page_size=`, `?category_id=`) |
-| POST   | `/datasets`                | upload (multipart: `file`, `file_type`, `category_id`, `code`, `name`, `description?`, `unit?`, `meta?` (JSON), `automated?` (bool)) |
+| GET    | `/datasets`                | paginated list of summaries (`?page=`, `?page_size=`, `?category_id=`, `?dg_id=`, `?start_date_from=`, `?start_date_to=`, `?end_date_from=`, `?end_date_to=`) |
+| POST   | `/datasets`                | upload (multipart: `file`, `file_type`, `category_id`, `code`, `name`, `description?`, `unit?`, `meta?` (JSON), `automated?` (bool), `dg_id?`, `start_date?`, `end_date?`) |
 | GET    | `/datasets/{id}`           | full dataset (geometry as GeoJSON, `bbox` for rasters) |
 | GET    | `/datasets/{id}.geojson`   | GeoJSON `FeatureCollection`; plain `vector` returns its geometry as a single feature with the extracted attribute table as top-level properties; `vector_with_kato` always ignores its own geometry and joins the `districts` table by KATO, mapping observations onto each polygon |
 | GET    | `/datasets/{id}/status`    | processing status; long-polls with `?current=<status>` (holds up to `?wait=` secs, default 25, max 60) |
 | GET    | `/datasets/{id}/download`  | download the stored file             |
-| POST   | `/datasets/{id}/mapping`   | set KATO column + year→date map (vector_with_kato) |
+| POST   | `/datasets/{id}/mapping`   | set KATO column + year→period map (vector_with_kato) |
 | GET    | `/datasets/{id}/observations` | paginated unpivoted values (`?kato_code=`, `?page=`, `?page_size=`) |
 | DELETE | `/datasets/{id}`           | delete (row + object)                |