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+# Technical Architecture
+
+## 1. Overview
+
+Single docker container Go application built on PocketBase as an embedded framework (https://pocketbase.io/). The binary contains:
+
+- PocketBase's HTTP/API server and admin dashboard;
+- the custom review dashboard;
+- the S3 poller and the PDF processing pipeline, executed through PocketBase's standard job scheduler (`app.Cron()`);
+- the declarative rule engine (JSONC config + `expr-lang/expr` expressions);
+- access to the self-hosted S3 storage via the MinIO Go SDK.
+
+Everything runs offline on the server infrastructure: no cloud APIs, no outbound calls other than to the S3 cluster.
+
+## 2. Stack
+
+| Component | Choice |
+|---|---|
+| Framework / runtime | PocketBase (Go framework, single binary) |
+| Language | Go |
+| Scheduling | `app.Cron()` (S3 poller, periodic tasks) |
+| S3 | MinIO Go SDK (`github.com/minio/minio-go/v7`) |
+| PDF text extraction | `github.com/ledongthuc/pdf` (native text, no OCR) |
+| Rule expressions | `github.com/expr-lang/expr` |
+| Regex | stdlib `regexp` (RE2); `regexp2` only if a lookaround proves indispensable |
+| Config validation | `santhosh-tekuri/jsonschema` (schema generated from structs via `invopop/jsonschema`) |
+| Database | SQLite (embedded in PocketBase) |
+| Full-text search | SQLite FTS5 index synchronized via hooks |
+
+## 3. S3 Layout
+
+A single versioned bucket:
+
+```
+inbox/       # drop zone for new PDFs (via Nextcloud / S3 client)
+pending/     # files awaiting human review
+files/       # renamed and filed files (configured prefix structure)
+```
+
+Nextcloud mounts the bucket as external storage: it is the user-facing interface for uploading, downloading, and sharing. The application only manipulates S3 keys, never a local filesystem for documents.
+
+## 4. Data Model (PocketBase Collections)
+
+### `documents`
+One row per processed PDF file:
+
+- `sha256`: the sha256 is a primary key as the system will reject as duplicate any file with the same hash
+- `original_filename`: original file name in `inbox/`
+- `extracted_text`: full extracted text (first page at minimum, stored for FTS)
+- `first_page_text`: first-page text (extraction base)
+- `size`: file size
+- `processed_at`: timestamp of the file initial process
+- `status`: `pending` | `processed` | `validated`
+- `updated_at`: timestamp of the last update to the file metadata
+- `history`: timestamped field/status changes (related `document_events` table)
++ all the fields declared by the user in the configuration file
+
+### `document_events`
+Append-only history: `document`, `event_type` (`classified`, `moved`, `corrected`, `validated`), `payload` as JSON, `timestamp`.
+
+
+## 5. Configuration File (JSONC)
+
+A `config.jsonc` file kept alongside the binary, reloaded on restart. Three sections:
+
+1. `fields`: declaration of fields to extract from the files:
+   - `name`: name of the field
+   - `type`: `string` | `number` | `boolean`
+   - `mandatory`: true | false
+
+2. `rules`: ordered list of rules that will be evaluated one after the other:
+   - `when`: an `expr-lang/expr` expression combining the extracted variables (e.g. `siren == "123456789" && has_iban`)
+   - `actions`: 
+       - `set`: set the value of one field
+	   - `stop`: stop the processing after this rule
+
+3. `templates`: naming templates per document type (e.g. `{sender}/{date}_{type}_{reference}.pdf`), with required-field validation — a template with a missing field renders the result invalid and routes the file to `pending/`.
+
+Validation on load: JSON schema, regex compilation, `expr` expression compilation against a typed context (`Document{FirstPageText, Sender, Siren, Iban, Confidence, ...}`). Full rejection if invalid, no partial processing.
+
+## 6. Processing Pipeline
+
+Executed by the PocketBase cron job (configurable interval, e.g. every minute):
+
+1. **Poll**: list objects in `inbox/` not yet processed (detection by key recorded in the database). Write-completion wait: an object is a candidate only if its size and ETag are stable across two consecutive polls.
+2. **Download**: fetch the object into memory (streamed), no disk writes.
+3. **Extraction**: first-page text (for rule evaluation) + full text (for FTS) via `ledongthuc/pdf`, including headers and footers
+4. **Rule evaluation**: rules evaluated one after the other; full log stored in `rule_trace` (built-in dry-run mode for debugging from the dashboard).
+5. **Validation**: a result is valid if and only if: all rules have been processed with no error, all mandatory fields are identified, and the naming template evaluated with no error. Otherwise: route directly to `pending/`.
+6. **Filing**:
+   - valid → copy to the new key (prefix + templated name), verify the target object exists, then delete the `inbox/` key only after the copy is confirmed; status `processed`. If a file with the same name already exists, append a number to the filename to avoid overwriting.
+   - invalid → move to `pending/`; status `pending`.
+7. **Indexing**: write the `documents` record, add an event to the file history including the original filename.
+
+The bucket is versioned: any `inbox/` object deletion creates a delete marker, the data remains recoverable.
+
+## 7. Review Dashboard
+
+Custom pages on the PocketBase instance:
+
+- File list: all files with status, filters, combined search over full text (FTS5) + metadata/fields.
+- Detail: PDF preview streamed from S3 through a dedicated route, along with extracted fields and `rule_trace` visible in debug mode.
+- Correction: forms to edit fields, type, and sender; correcting a `processed` file re-files it (copy to the new key, delete the old one, history preserved).
+- Validation: transition to `validated`; `processed` files can also be validated.
+- Rule management: config display, dry-run on an existing document, reload.
+
+Authentication: single account via PocketBase's standard auth.
+
+## 8. Error Handling
+
+- Transient S3 error: retry with backoff, the object stays in `inbox/` and will be polled again.
+- Unreadable PDF: `pending` status, reason recorded.
+- Target key conflict (two files renamed identically): automatic incremental suffix, event logged.
+- Invalid config: error message displayed and the application does not start.